It was a cool night for Havana, with the temperature falling into the mid-70s, and the diplomat and his family were feeling very good about their assignment to Cuba. They were still settling into their new home, a comfortable, Spanish-style house in the lush enclave that had been called “el Country Club” before wealthy families abandoned it in the early years of the revolution. “We were just thrilled to be there,” the diplomat recalled. “The music, the rum, the cigars, the people — and a very important moment for diplomacy.”

Eight months earlier, in March 2016, President Barack Obama had swept into town to commemorate the two countries’ historic rapprochement, vowing to bury “the last remnant of the Cold War in the Americas.” Now, weeks after the election of Donald Trump, that entente was suddenly doubtful. Fidel Castro had just died, opening a new chapter in the Cuban saga. The diplomat could hardly have imagined a more fascinating time to arrive.

As the sun slid into the Florida Straits on that late-November evening, the diplomat folded back the living room doors that opened onto the family’s new tropical garden. The warm night air poured in, along with an almost overpowering din. “It was annoying to the point where you had to go in the house and close all the windows and doors and turn up the TV,” he recalled. “But I never particularly worried about it. I figured, ‘I’m in a strange country, and the insects here make loud noises.’”

A few nights later, the diplomat and his wife invited over the family of another American embassy official who lived next door. Around dusk, as they chatted on the patio, the same deafening sound rose from their yard again.

“I’m pretty sure those are cicadas,” the first diplomat said.

“Those are not cicadas,” his neighbor insisted. “Cicadas don’t sound like that. It's too mechanical-sounding.”

One of the U.S. diplomats affected by the health incidents reportedly lived in this home in Havana. (Courtesy of NBC News)

The colleague had been hearing the same noises at home, sometimes for an hour or more at a stretch. After he complained to the embassy housing office, a couple of Cuban maintenance workers were dispatched to look around. They checked for electrical problems and scanned the yard for strange insects, but they left without finding anything out of place. In February, the nightly racket finally began to fade. Then it went away altogether.

It was not until a Friday in late March that the diplomat realized he might be facing something more dangerous than bugs. At work that day, an embassy colleague with whom he was friendly took him aside and said he was leaving Cuba right away. A fit-looking man in his thirties, the colleague said he had just been in Miami, where medical specialists found he had a series of problems including a serious hearing loss. In late December, he said, he had been struck by a strange, disturbing phenomenon — a powerful beam of high-pitched sound that seemed to be pointed right at him. The following Monday, the diplomat’s friend played him a recording of the noise: It sounded a lot like what the diplomat had heard in his backyard.

The diplomat, who agreed to discuss his experience on the condition he not be named, said neither he nor his wife had felt any signs of illness or injury. But within days, they, too, would be on their way to Miami to be examined by medical specialists. Along with 22 other Americans and eight Canadians, they would be diagnosed with a wide array of concussion-like symptoms, ranging from headaches and nausea to hearing loss. They would also find themselves caught up in an extraordinary international dispute, one that the Trump administration would use to sharply reverse the course of U.S. relations with Cuba.

Even in a realm where secrets abound, the Havana incidents are a remarkable mystery. After nearly a year of investigation that has drawn on intelligence, defense and technology expertise from across the U.S. government, the FBI has been unable to determine who might have attacked the diplomats or how. Nor has the bureau ruled out the possibility that at least some of the Americans weren’t attacked at all. Officials who have been briefed on the inquiry described it as having made strikingly little progress in answering the basic questions of the case, with frustrated FBI agents reporting that they are running out of rocks to overturn.

Those frustrations have roiled the U.S. national-security community, putting the FBI increasingly at odds with the CIA over the case. In early January, after more than eight months of analysis, the bureau ruled out its initial hypothesis that the Americans were targeted with some type of sonic device. That left the FBI without a weapon, a perpetrator or a motive, and still struggling to understand how the diplomats could have been hurt or fallen ill. Intelligence officials, for their part, have continued to emphasize a pattern they see as anything but coincidental: The first four Americans to report being struck by the phenomenon — including the fit-looking man in his 30s — were all CIA officers working under diplomatic cover, as were two others affected later on. The CIA and other agencies involved in the investigation also have yet to concur with the FBI’s conclusion about sonic technology.

More broadly, the Cuba problem has raised questions within the national security community about how the Trump administration is using intelligence information to guide its foreign policy. At a time when the White House has vowed to act more forcefully against North Korea, Iran and other threats, some officials see the Cuba problem as yet another lesson in the dangers of using intelligence selectively to advance policy goals. “Trump came in opposing better relations with Cuba,” said one national security official who, like others, would discuss the case only on the condition he not be named. “The administration got out in front of the evidence and intelligence.”

A ProPublica investigation of the case, based on interviews with more than three dozen U.S. and foreign officials and an examination of confidential government documents, represents the first detailed public account of how the Cuba incidents unfolded. Although the State Department has generally emphasized similarities in the medical files of the 24 affected Americans, officials and documents consulted for this story indicated that the nature and seriousness of the patients‘ symptoms varied rather widely. The experiences that precipitated their illnesses were also quite different, officials said, and the experiences and symptoms of the eight Canadians differed from those of the Americans.

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Many U.S. officials who have dealt closely with the problem — including several who asserted that it has been distorted for political purposes — said they remain convinced that at least some of the Americans were deliberately targeted by a sophisticated enemy. Medical specialists who reviewed the patients‘ files last summer concluded that while their symptoms could have many causes, they were “most likely related to trauma from a non-natural source,” the State Department medical director, Dr. Charles Rosenfarb, said. “No cause has been ruled out,” he added. “But the findings suggest this was not an episode of mass hysteria.”

Yet it appears that secrecy, psychology and politics may all have played some part in how the phenomenon spread through the staffs of the two Havana embassies. Administration officials have been reluctant to discuss psychological factors in the case, in part because they fear offending or antagonizing the stricken diplomats (many of whom already feel badly treated by the State Department leadership). But as the mystery has deepened, U.S. investigators have begun to look more closely at the insular, high-pressure world of the Havana embassy, and they have found a picture that is far more complex than the rhetoric and headlines have suggested.

Despite the many unanswered questions, Trump administration officials have repeatedly blamed Raúl Castro‘s government for failing to protect the diplomats, if not actually attacking them. Early last fall, the State Department withdrew more than half of the diplomatic staff assigned to Havana, while ordering a proportional number of Cubans to leave Washington. The department also warned U.S. citizens they could be “at risk” of attack if they visit the island. “I still believe that the Cuban government, someone within the Cuban government, can bring this to an end,” Secretary of State Rex Tillerson said last month.

What the Diplomats Heard via the Associated Press YouTube account

Such assertions have outraged the Cuban leadership. Since early last year, U.S. officials said, Castro and his senior aides have insisted they had nothing to do with the incidents and would help in any way they could to investigate and stop them. The FBI team has found no evidence of Cuban complicity in the incidents, officials said, and has privately emphasized the government‘s cooperation with its investigators. Tillerson‘s statements notwithstanding, some State Department officials have also told members of Congress privately that they have assessed the Cubans‘ denials of involvement to be credible, officials said. “They believe the Cuban government wants better relations with the United States,” one Senate aide said.

The other obvious suspect has been Russia, which intelligence analysts have seen as having both a possible motive and the possible means to carry out such attacks. The Putin government has harassed U.S. diplomats routinely in Moscow and sometimes abroad; during the Obama administration, it appeared determined to disrupt American foreign policy around the world. Russia also has a capacity to engineer sophisticated new weapons and a longstanding security alliance with Cuba. But investigators have not found even significant circumstantial evidence of a Russian hand in the incidents, officials said, and some analysts doubt Russia would imperil its relationship with Cuba by so brazenly undermining one of its key foreign policy goals.

While the mystery continues, U.S. policy toward Cuba hangs in the balance. With Castro scheduled to step down from the presidency in April, Washington is represented in Havana by only a skeleton staff at a potentially critical moment of transition. American travel to and business with the island have fallen sharply in recent months, and the processing of visas for Cubans wanting to emigrate to the United States has plunged, calling into question the fulfillment of a longstanding migration agreement between the two countries. The Trump administration may also have limited its options: On March 4, the State Department will face a deadline to either send its diplomats back to Havana or make permanent staff reductions. But the Secretary of State, who reportedly made the decision to pull out the diplomats, has shown no signs of changing his position.

“We don’t know how to protect people from this, so why would I do that?” Tillerson told the Associated Press when asked about returning diplomats to Cuba. “I will push back on anybody who wants to force me to do that until I‘m convinced that I’m not putting people in harm‘s way.”

Cuban President Raul Castro talks with Vice President Miguel Diaz-Canel during the year-end parliamentary session in Havana on Dec. 21, 2017. (Jaime Blez/AFP/Getty Images)

In the crossfire of accusations, ordinary Cubans might be forgiven for wondering if they have been transported back in time. As the country prepares to be led for the first time in almost 60 years by someone not named Castro, a tectonic shift that could profoundly affect how it is governed, cold war rhetoric has again filled the air. The next-generation Communist leader who is expected to succeed Raúl Castro, Vice President Miguel Díaz-Canel, 56, is among those who have warned of yet another imperialist plot against Havana. They are “incredible fairy tales without any evidence,” he said of the Trump administration’s claims, “with the perverse intention of discrediting Cuba‘s impeccable conduct.”

Key Locations in the Havana Attacks The first three diplomats who reported suffering what they thought were “sonic attacks” in November and December 2016 lived in upscale neighborhoods in Havana’s western suburbs. Two other incidents later occurred the following April at the Hotel Capri, near the center of the city. Hannah Birch/ProPublica

The first two incidents occurred around Thanksgiving weekend of 2016, which coincided with the death of Fidel Castro on Nov. 25. During the nine days of official mourning that followed, neither American official told the embassy’s leadership what they had experienced. But both men, intelligence officers working under diplomatic cover, would later say they heard sharp, disorienting sounds in their homes at night. At least one of them would later tell investigators the noise had seemed oddly focused, officials said. Moving out of the way or into another room, it seemed almost to disappear.

If the stories sounded like science fiction, the CIA’s Havana station and the embassy leadership suspected something more mundane. Since the United States and Cuba restored limited diplomatic relations in 1977, reopening their embassies as “interests sections” in each other’s capitals, the Cubans kept a constant, often aggressive watch over American diplomats in Havana. Diplomats might come home to find a window opened, or a television set turned on (often to government news), or their belongings slightly but obviously rearranged. Some part of the game — including more provocative actions like smearing dog feces on the handles of diplomats’ car doors — was considered almost routine. There was also some noted reciprocity from the American agents who trailed Cuban diplomats around Washington.

During periods of particular tension with Washington, the Cubans sometimes went further. In the early and mid-1990s, American diplomats who met with Cuban dissidents or otherwise annoyed the government occasionally returned from meetings to find their car tires punctured. In the mid-2000s, as the Bush administration openly pursued efforts to subvert the Castro regime, Cuban harassment of the 51 American diplomats then stationed on the island ranged from delays in the release of food shipments to “the poisoning of family pets,” the State Department’s inspector general wrote in a 2007 report.

Jeffrey DeLaurentis, the top U.S. diplomat in Cuba, delivers a speech during the U.S. Embassy building reopening ceremony in Havana on Aug. 14, 2015. (AFP Photo/Adalberto Roque)

The man who headed the American diplomatic mission in late 2016, Jeffrey DeLaurentis, knew that history of harassment well, officials said. A measured, laconic career diplomat with an air of hardened patience, DeLaurentis had taken over as the chargé d’affaires in the summer of 2014, bringing more Cuban experience than perhaps any other senior official in the U.S. government. He had done previous tours in Havana as both a consular officer and a political officer, with a stint in between managing Cuban affairs on the National Security Council staff. After Obama announced a plan to normalize diplomatic relations with Cuba in December 2014, he nominated DeLaurentis to be Washington’s first ambassador to Havana since 1961, when President Eisenhower severed diplomatic relations. (Although his confirmation was blocked by Senator Marco Rubio of Florida, who argued that Cuba should demonstrate greater respect for human rights before the post was filled, DeLaurentis remained as the chargé d‘affaires.)

Obama’s visit in March 2016 had left Cuban leaders ambivalent about the hand of friendship he extended: Fidel Castro, ailing and almost 90, stirred from his retirement to attack the American president‘s “syrupy words,” and what he saw as an insidious plea for Cubans to forget the Americans‘ dark history with the island. At a Communist Party congress that April, Raúl Castro and others peppered their rhetoric with references to “the enemy” to the north. Diplomats also noted some palpable discomfort among senior Cuban officials with the burst of capitalist bling that marked the easing of U.S. commercial restrictions — a Chanel fashion show, a free Rolling Stones concert, the brief takeover of Havana streets to film scenes for a new “Fast and Furious” movie.

But in the last months of 2016, official Cuban hostility toward the American diplomats in Havana was hovering somewhere near a 50-year low. No serious harassment had been reported for at least a few years, officials said. Most close analysts of Cuba believed the ruling party had forged a solid consensus for ending hostilities with the U.S. Fidel Castro’s last, angry diatribe notwithstanding, U.S. officials told ProPublica that he had been consulted on the rapprochement and given his approval.

While Cuban officials were notably slow to move forward with many of the proposed American business deals that poured in, they did plod ahead with work on bilateral agreements on law-enforcement cooperation, environmental protection, direct mail service and other matters. “Of course, there is a range of preferences within the regime on the speed and depth of reform,” said Fulton Armstrong, a former senior CIA analyst who handled Cuba issues on both the National Security Council staff and the National Intelligence Council. “But the debate is about the pace; there is no alternative to the Raúl strategy.”

Barack Obama and Raul Castro attend an exhibition game between the Cuban national team and the Major League Baseball team Tampa Bay Devil Rays at the Estadio Latinoamericano March 22, 2016, in Havana, Cuba. (Chip Somodevilla/Getty Images)

The Cubans’ attention became more focused after the Nov. 8 presidential vote, American officials said. Although Trump had vowed during his campaign to renegotiate Obama‘s “very weak agreement“ with Havana, the Castro government had seemed to discount the possibility that he could be elected. Once Trump was elected — and with Obama administration officials urging the Cubans to consolidate improvements in the relationship — the Cuban government hurried to conclude work on pending agreements before the Jan. 20 inauguration.

It was during that same period between the election and the inauguration that the first U.S. intelligence officers were struck by what they described as strange noises. The initial three victims lived in the upscale neighborhoods of Havana’s western suburbs. Fidel Castro kept a home in one of those neighborhoods, Cubanacán, as do Vice President Díaz-Canel and other members of the island‘s most-privileged elite. The elegant old mansions and tropical-suburban homes of the enclave are also favored by senior foreign diplomats and business executives. There is relatively little car or pedestrian traffic, and a considerable presence of private security guards as well as the Cuban police.

Although the first two officers would later report having first heard strange sounds in their homes back in late November, it was not until the end of December that the first victim sought help at the small medical clinic inside the embassy. That officer — the fit younger man in his 30s — came with a more serious complaint: He had developed headaches, hearing problems and a sharp pain in one ear, especially, following a strange experience in which something like a beam of sound seemed to have been directed at his home.

The younger man’s trauma was reported to DeLaurentis and the embassy‘s diplomatic security chief, Anthony Spotti, on Dec. 30, State Department officials said, and followed by word that the two other CIA officers had experienced something similar about a month before. But inside the modernist glass-and-concrete chancery building that rises up along Havana’s iconic seawall, the Malecón, both the intelligence officials and senior diplomats guessed that the noises were “just another form of harassment” by the Cuban government, one official said. They also seemed carefully targeted to CIA officers working under diplomatic cover. If members of Cuba’s state-security apparatus did not know the men were intelligence officers, they would probably have suspected them anyway, the Americans believed.

Fidel Castro's funeral procession in Santa Clara, Cuba, Dec. 1, 2016. Castro died during the period that the mysterious incidents affecting U.S. diplomats began. (Tomás Munita, special to ProPublica)

The incidents were discussed discreetly among members of the embassy’s “country team,” the group of roughly 15 senior diplomats that would often meet daily to discuss significant issues. But, because of counterintelligence concerns, they were kept secret from most of the other American personnel — about 32 other diplomats and eight Marine guards — a decision that was later criticized by some of those who became sick. “We have security officers at every embassy and they give us constant updates,” one diplomat said. “Somebody gets pick-pocketed, somebody got their car broken into ... And then somebody got attacked by this mystery weapon and they didn‘t tell us?”

By mid-January, after the other two intelligence officers also sought medical attention at the embassy, the matter began to take on a more ominous cast, several officials said. Around the time that the first intelligence officers were sent to the U.S. for treatment on Feb. 6, the wife of another embassy staffer, who lived near the Havana coastline in the neighborhood of Flores, reported hearing similar, disturbing sounds, two officials familiar with her account said. The woman then looked outside and saw a van speeding away. The vehicle had apparently come from the same end of the street on which there was a house that was thought by U.S. officials to be used by the Cuban Interior Ministry. The officials acknowledged that the report was vague and uncertain. Yet they said it also constituted one of the more significant pieces of circumstantial information they had about the incidents.

In Havana, officials said, senior members of the embassy staff argued to their counterparts in Washington that they should formally protest the incidents to the Cuban government. Given the uncertainties, others thought they should try to gather more information before lodging such a complaint. Although it was a matter of concern at both the State Department and the CIA, it is unclear whether it was raised to the National Security Council staff before the decision to protest was made (one former senior official said it was not). Nor, officials said, was Secretary of State Tillerson informed of the situation until days after the department‘s acting assistant secretary for Western Hemisphere affairs, Francisco Palmieri, finally called in the Cuban ambassador in Washington, José Ramón Cabañas, to present a diplomatic note of protest on Feb. 17.

The Cuban government responded promptly. A few days later, officials said, DeLaurentis was called to a meeting with Josefina Vidal, the senior diplomat who had led the Cuban team that negotiated the normalization of relations with the U.S. (DeLaurentis declined to comment, referring questions about the Havana incidents to the State Department.) Vidal was joined by other officials from the Interior Ministry, which controls the country’s foreign-intelligence and internal-security apparatus. The Cuban security officials questioned DeLaurentis about the incidents, what the diplomats had experienced, what symptoms they had suffered and what other circumstances might shed light on the episode, officials said.

On Feb. 23, less than a week after the U.S. démarche to the Cuban government, DeLaurentis accompanied two visiting U.S. senators, Richard Shelby of Alabama and Patrick Leahy of Vermont, to see President Raúl Castro at the Palace of the Revolution. During the conversation, officials said, Castro mentioned that he had something to discuss with the chargé, and when the meeting concluded, he asked DeLaurentis to stay behind. During what officials described as a fairly brief but substantive conversation, Castro made it clear that he was well aware of the incidents and understood that the Americans saw them as a serious problem. His response, one State Department official said, was “We should work together to try to solve it.”

The Americans’ meetings with Cuban diplomatic and security officials continued. The Cubans said they would bolster security around the homes of American diplomats, adding police patrols and installing closed-circuit television cameras in some areas. In a more unusual step, the Cubans also allowed a team of FBI investigators to come to Havana to investigate for themselves, building on improvements in the law-enforcement relationship that were formalized with a bilateral agreement in late 2016. (An FBI spokeswoman said the bureau would not comment on details of the investigation.)

From the start, U.S. officials were themselves reluctant to share information with Havana about the incidents. The Cubans asked to interview the Americans identified as victims; the State Department refused. The Cubans asked for detailed medical information about their injuries; the State Department demurred, citing privacy concerns. “You could not rule out” the Cuban government’s possible involvement in the incidents, one department official said. “When you are dealing with a possible perpetrator, one is careful.”

While the first embassy staff members were sent to be evaluated by specialists at the University of Miami’s Miller School of Medicine, officials in Washington also began to look more widely at what might be causing their symptoms. Initially, U.S. intelligence officials hypothesized that either the Cuban government or some other foreign regime — possibly with Cuban participation — had created a new kind of Long-range Acoustic Device, or L-Rad, enabling them to somehow focus and direct powerful sonic waves of the sort that are used by police agencies to disperse crowds, or by cargo ships to drive away pirates.

But the physics were puzzling to experts inside and outside of government. The incidents had mostly taken place at night, inside victims’ homes. Whatever sonic or directed-energy weapon was used seemed to have penetrated walls and windows. Yet others living in the immediate vicinity apparently heard nothing out of the ordinary. With known L-Rad technology, sound waves generally radiate out from the device. No one seemed to understand how it could be focused in an almost laser-like fashion and still penetrate hard surfaces.

After a lull of several weeks, the incidents began again — and there were more of them. One woman was struck in her apartment. Other diplomats were hit in their homes in the western suburbs. The differing circumstances only complicated the picture, but the effects of the phenomenon became clearer: The first patients examined in the U.S. were all found to have concrete medical symptoms, and in the case of the younger man, they were fairly serious.

On Friday, March 24, the diplomat who had first heard the noises in his backyard around Thanksgiving encountered the younger man at work and heard about his frightening diagnosis in Miami. Doctors said the man had serious damage to the small bones inside one of his ears, among other issues, and would need to wear a hearing aid. The next Monday, he played the diplomat a recording of the noise with which he had been targeted. The diplomat was stunned: It sounded much like the noises that he and his family had heard from their garden for several months.

A day later, the diplomat went to see DeLaurentis in the spacious, fifth floor ambassadorial suite that looks out over the Malecón, officials familiar with the episode said. The diplomat explained that he, too, had been exposed to strange sounds that seemed similar to what the younger man had experienced. DeLaurentis said he and others who knew about the incidents believed they were confined to a “small universe of people” whom the Cubans probably suspected of doing intelligence work, whether they were CIA officers or not. The diplomat wasn’t reassured, and he suggested that others would not be, either. “You need to call a meeting,” the diplomat told DeLaurentis. “The rumor mill is going mad.’”

The next day, March 29, DeLaurentis gathered about four dozen members of the embassy’s American staff — everyone in the building who had a security clearance. This time, after surrendering their cell phones, they crowded into a windowless conference room that had been outfitted as a Sensitive Compartmented Information Facility, or SCIF (pronounced “skiff.”) It had already been more than a month since DeLaurentis delivered his formal complaint to the Cuban government, but most of the people in the room were hearing about the incidents for the first time.

According to three officials who attended the meeting, DeLaurentis calmly laid out the basic details of what some of the diplomats had experienced. There was much they still did not understand about what had happened and who might be behind it, he said, but investigations were underway, and the Cuban authorities were taking steps they had promised to increase the diplomats’ security. He encouraged anyone who thought they might have been exposed, or who had any information that could be relevant to contact him or speak with the embassy’s security officer. Medical specialists were available to examine anyone who showed signs of a problem.

If DeLaurentis was hoping to calm his troops, he appears to have been only modestly successful. Part of the problem, diplomats said, was that he concluded the meeting by asking the assembled staff to avoid talking about the situation outside the secure confines of the embassy, even with their families. Although the matter was still classified, that request struck at least some of them as unreasonable, even outrageous. “We thought that was nuts,” said one official who attended the meeting. “There were family members who were attacked at home. How could we not tell them to watch out for this?”

Concerns among the staff and their dependents about their health exploded. Within barely a month, diplomats reported a flurry of new incidents. By the end of April, more than 80 diplomats, family members and other personnel — a very high proportion for a mission that included about 55 American staff — had asked to be checked out by the Miami medical team. That group was led by an ear, nose and throat specialist, Dr. Michael E. Hoffer, who has worked extensively with military veterans who suffered vestibular trauma from explosions and fighting in Afghanistan and Iraq. Based on examinations in both Miami and Havana, it quickly identified almost a dozen new cases — nearly half the number that would eventually be confirmed.

The affected diplomats experienced a wide range of sensations: Some heard sharp, piercing noises or a cicada-like buzz. Others felt concentrated “beams” of sound or auditory vibrations like those from the half-open window of a fast-moving car. Still others heard no sound at all. According to a one-page summary of the cases that was jointly prepared for the Cuban government by the State Department’s bureaus of Medical Services and Western Hemisphere affairs, “Some voiced feeling shocked or shaken by the exposure, or awoken (sic) from sleep, and others described a more gradual onset of symptoms that continued for days to weeks afterwards.”

Amid the fear that gripped many, some embassy staff came forward saying they might have heard or felt similar phenomena, but were found after being interviewed not to require medical attention. Among the first 20 people examined by specialists in Havana and Miami, nine were found to have no discernable symptoms, while nine others had “moderate” effects like headaches, nausea, tinnitus and dizziness. Only two had what were termed “the most severe” effects, including the younger man who reported the first symptoms in late December.

The landmark Hotel Capri in Havana, where two visiting U.S. officials reported that sharp, piercing noises shook them in their rooms in April 2017. (AP Photo/Desmond Boylan)

After another lull of a few weeks, a disturbing new incident occurred in late April at the Hotel Capri, a 19-story landmark that was once a favorite of various Mafia dons and the actor Errol Flynn. Now run by a Spanish firm, the hotel was one of several used by the U.S. Embassy to put up diplomats and official visitors. Around April 21, an embassy staffer who was staying there during renovations on his apartment was shaken at night by a piercing noise in his room. A day or two later, an American doctor who had just flown in with the University of Miami team experienced a similar phenomenon. Both men had rooms with relatively large windows, an official said, yet other guests apparently nearby heard nothing.

This time, the embassy responded to the Cubans more vehemently. The diplomats who had been affected earlier had been living in their homes for some time. But the two new Americans who reported being struck were in hotel rooms that were presumably known only to a small number of U.S. and Cuban officials, and the hotel staff. The doctor had just arrived on the island a day or two earlier. “Who knew that he was there?” DeLaurentis demanded of the Cuban foreign ministry, according to one official familiar with the exchange. “The U.S. government. And the Cuban government.”

Within the Trump administration, anger over the incidents grew. On May 20, Cuba’s independence day, the president issued a statement warning that “cruel despotism cannot extinguish the flame of freedom in the hearts of Cubans.” Three days later, the State Department expelled two Cuban diplomats in Washington who had been identified by the U.S. as spies. The expulsions were not made public, and no word of the acoustic mystery in Havana leaked to the news media. Yet even as diplomats and law-enforcement officials from the two countries continued to collaborate on the investigation in a limited, low-key way, the relationship veered back toward confrontation.

The Trump administration was by then finalizing plans to undo Obama’s rapprochement. Exactly what it would roll back to was uncertain; Trump suggested that the Cubans had gotten off easy on human rights, but he offered no particular rebuttal to the argument made by State Department officials and others in the government that greater engagement with Cuba was the most effective way to promote economic and eventually political liberalization there. Some American business groups and more moderate Cuban-American political groups also pushed for continued engagement. But in a new administration that had not filled senior Latin America posts at the State Department or on the NSC staff, many officials said there was a vacuum of policy leadership on the issue.

Senator Marco Rubio speaks about the State Department’s handling of the health problems suffered by U.S. Embassy officials in Havana during a Senate Foreign Relations subcommittee hearing in Washington, D.C., Jan. 9, 2018. (Kevin Lamarque/Reuters)

That vacuum was filled above all by the former campaign rival whom Trump had disparaged as “Little Marco.” Starting soon after the administration’s first closed-door intelligence briefing to Congress on the Havana incidents, Rubio pushed for a tougher response, officials said, and also advocated a series of hardline proposals to the broader Cuba policy. The White House “asked for my input on basically every issue in Latin America and the Western Hemisphere and ... we’ve been engaged with them and they’ve been very open,” Senator Rubio told McClatchy newspapers. “In some ways, the fact that they didn‘t come in with preconceived ideas of what to do has created the space for that debate to occur.”

On June 16, Trump traveled to Miami to announce he was “canceling the last administration’s completely one-sided deal with Cuba.” Although the changes fell short of that, Trump ordered government agencies to revise regulations on travel and business to prohibit any transactions with hotels, restaurants, stores and other companies tied to the large tourism and business operations of the Cuban military. Americans other than Cuban-Americans would not be allowed to travel on their own for general tourism purposes, but only with organized educational and other groups on pre-set itineraries. Any further improvements in the bilateral relationship, Trump said, would be contingent on human rights improvements in Cuba. “Now that I am president,” Trump promised, “we will expose the crimes of the Castro regime!”

In Havana, the diplomat who had first heard the noises in his garden was sent off to Miami in early April for medical testing with a cluster of other embassy personnel. He and his wife would return only to pack their things. Before leaving Cuba, though, he stopped to say goodbye at the home of one of his Canadian neighbors and tell him a bit about why they had to leave. The Canadian diplomat was worried: His family had been hearing similar sounds, he said. Could they have caused a mysterious nosebleed his son had suffered? Or headaches his wife had had?

In late April, DeLaurentis had invited over a small group of ambassadors from countries closely allied with the U.S. — Canada, Britain, France and others — to let them know what had been happening to his staff and ask if anyone else had experienced something similar. Other than one report from a French diplomat that was quickly discounted, the only significant response came from the embassy of Canada. In early May, the Canadian ambassador, Patrick Parisot, gathered the 18 diplomats on his staff to relay the Americans‘ warning and ask if anyone had heard strange noises or suffered unusual illness. Several people reported back, a Canadian official said, including one (apparently the American diplomat‘s neighbor) who said he had heard strange noises in his garden back in March.

As at the American embassy, fears about what was happening spread quickly through the Canadian staff. In all, 27 Canadian diplomats, spouses and children, representing 10 of the embassy‘s families, sought medical attention. Of those, eight people from five families — including two children — would be diagnosed with symptoms that were milder than those of almost all the American patients: nosebleeds, dizziness, headaches and insomnia. All would recover fairly quickly.

In general, a Canadian official involved with the case said, the experience that triggered the Canadian diplomats’ symptoms was quite different from those reported by the Americans. In addition to the Canadian diplomat who said he had heard noises in his garden, members of another diplomatic family reported one day in June that they had heard a sudden, twanging sound, like a piece of sheet metal being waved; one family member later became ill. But the other six Canadians who were sickened did not hear or experience anything similar.

“In most cases, there weren‘t really attacks that we could point to,” the Canadian official said. “The American experience was all about acoustic events and people feeling ill, and we had people feeling ill with limited connections to acoustic events.”

The Canadian foreign ministry also managed the issue very differently from the Americans, avoiding any criticism of the Cuban government. The ministry said it had no plans to reduce diplomatic staffing levels in Havana, and it quickly replaced the three embassy families that chose to return home because of the problem. The government also said the Royal Canadian Mounted Police had received all the assistance it asked of the Cuban government. “The Cubans are pretty attached to the 1.2 million Canadian tourists who come to Cuba every year, so they‘ve got a pretty strong incentive to nip this in the bud,” the official said. “They’ve been very proactive in trying to help us.”

However, the Canadian police have made virtually no real progress in their investigation, the official said, despite help from both the Cuban security forces and the FBI. After consulting with intelligence and technology experts, U.S. and Canadian security officials have recommended that diplomats and their families move away as quickly as possible from any unusual sound they might hear. The U.S. embassy also handed out high-frequency recorders so diplomats could record the noises, and relocated some of them from homes where the sounds or vibrations had been felt repeatedly.

The FBI investigative team, which has included members of a Miami-based unit that investigates crimes against U.S. citizens in Latin America, has visited Cuba four times since May. The group has interviewed diplomats and other officials of both countries, examined the homes and hotels where incidents took place, and conducted other inquiries. Their assessments have fed into elaborate matrices comparing the physical circumstances of the reported incidents with the sensations that the Americans described and the medical problems they later suffered. They also contributed to the still-secret report of the bureau’s Operational Technology Division on Jan. 4 that concluded that the Americans’ symptoms were not caused by some type of sonic device. (A State Department diplomatic security official, Todd Brown, said the investigators are still considering the possibility that sound was used to mask some other harmful agent or technology.)

A resident looks out a window of his home in Havana. (Eliana Aponte/Bloomberg via Getty Images)

The Havana investigation has also involved a wide range of U.S. scientific and technological agencies, including the CIA‘s Directorate of Science and Technology and the Pentagon‘s Defense Advanced Research Projects Agency. But officials said it is not clear that any of those have made significant progress, either. In addition to ultrasonic and infrasonic technologies, they have examined other directed-energy technologies. Some inquiry has also focused on the possible use of microwaves, harking back to the Moscow Signal, an episode from the 1970s in which Soviet intelligence beamed microwave signals into the U.S. embassy in Moscow to activate a passive receiver hidden in the office of the United States ambassador, officials said. Americans in the embassy were later reported to have been sickened by the phenomenon, but their symptoms did not closely resemble those suffered by diplomats in Cuba.

In interviews, former U.S. intelligence officers said they were also skeptical of the idea that the U.S. diplomats in Cuba might have been subjected to some new surveillance effort gone awry. Because the Cubans have always kept close tabs on American diplomats in Havana, they said, the security forces generally know they have little to fear from the recruitment or intelligence-gathering efforts of American spies stationed on the island. The intelligence experts also noted that the monitoring of diplomats at home is a labor-intensive task that would likely be reserved for the most important targets.

“In my experience, those operations at residences mean you end up sifting through a lot of trash,” said Charles S. (Sam) Faddis, a former senior CIA operations officer. “The product you get is filled with extraneous noise, daily life, every marital disagreement, the sounds of the TV, the kids, the dog. It seems like a lot of effort for that kind of target.”

Among the scientists whom the FBI team has sought out was Allen Sanborn, a biologist at Barry University in Miami Shores, Florida, who has spent 30 years studying cicada populations in Latin America and elsewhere. Dr. Sanborn said that while cicadas do make very loud noises, “it’s doubtful they could cause injury in Cuba because of the size and species.” He estimated that the Cuban cicada could reach a deafening 95 decibels at a distance of about 20 inches, but emphasized that the sound-pressure level would drop six decibels with every doubling distance. So, at 40 inches away, the sound intensity would fall to 89 decibels, and at 80 inches it would fall to 83 decibels, and so on. “It wouldn‘t really hurt you unless it was shoved into your ear canal,” he said in an interview.

The four FBI agents who came to Dr. Sanborn’s home for the interview asked him a series of questions about insect calls in general and cicadas in particular. Then, they asked him to listen to about a dozen recordings made by American diplomats in Havana who had experienced what they thought at the time was some type of sonic attack. Some were shorter, some longer, Dr. Sanborn said, but all were about the same frequency and seemed to be the same sort of sound. He cautioned that the recordings were not of an extremely high quality, but he offered the agents his best judgement.

“The three possibilities are crickets, cicadas and katydids,” he said. “They sounded to me like cicadas.”

Dr. Sanborn said he gave the agents a couple of academic papers he has written that include analyses of the temporal patterns and spectral frequency of various cicada calls, but has not heard from them again.

Only the medical side of the investigation has produced somewhat more conclusive results. In early July, the State Department’s medical services bureau assembled a panel of neurological, otolaryngological and other experts to review the medical files of the Havana patients. The physicians allowed that at least some of what the diplomats had experienced could have come from other sources, including “viral illnesses, previous head trauma, aging, and even stress,” Dr. Rosenfarb said. But, he added, the experts’ consensus was that “the patterns of injuries that had so far been noted were most likely related to trauma from a non-natural source.”

There had been no new attacks since April, although some of those affected only reported their symptoms weeks or months later. But then, around Aug. 21, two more incidents were reported, at least one of them at the Hotel Nacional, a fortress of 1930s luxury not far from the Capri. Shortly after doctors confirmed on Sept. 1 that the two patients showed symptoms associated with the incidents, the State Department put the Havana mission on a “voluntary departure” status, allowing any of those serving there to leave with their families. The reason the department gave for the order was the impending Hurricane Irma, which raged across the north coast of the Island a few days later.

But many of those who left temporarily would not return, or would go back only to gather their belongings. In a sweeping, punitive action on Sept. 29, the State Department ordered home 24 of the 47 diplomats assigned to Havana, including all of those with families. It effectively shut down the embassy‘s consular section except for emergency services. The department then ordered 15 more Cuban diplomats to leave Washington, including some involved in visa-processing and commercial affairs.

The department still did not accuse the Cuban government of direct involvement in what it called the Havana “attacks.” But it warned Americans not to travel to the island in terms more ominous than those sometimes used for some countries wracked by political upheaval, and caveats it offered about the continuity of diplomatic relations were quickly lost in the surging rhetoric. “There is no way that someone could carry out these number of attacks, with that kind of technology, without the Cubans knowing about it,” asserted Senator Rubio, who had again been urging a more forceful response. “They either did it, or they know who did it.”

The Cubans, Trump declared, “did some very bad things.”

President Donald Trump, after signing changes in Cuba policy on June 16, 2017, in Miami, Florida. Trump restricted U.S. travel and business on the island. (Joe Raedle/Getty Images)

It was a script that the Cuban government seemed to recognize. The foreign minister, Bruno Rodríguez, who had earlier called Trump’s Miami speech in June “a grotesque spectacle,” emphasized one point above others: The United States had presented no evidence whatsoever that the Cubans had done anything but try to help investigate the problem. Although the United States has suggested that Cuba have failed to live up to its responsibilities to protect foreign diplomats under the Vienna Conventions, Cuban officials have emphasized that Washington has not cited any specific actions the Cuban government has failed to take toward that end.

“Cuba has taken absolutely no measures at all against the United States,” Rodríguez said, referring to American sanctions. “It does not discriminate against its companies. It invites its citizens to visit us, promotes dialogue and bilateral cooperation.” The actions taken by the United States, he added, “can only benefit the sinister interests of a handful of people.”

Foreign-policy experts inside and outside the government generally agree that the Havana incidents seem to run counter to the interests of the Castro government. “The Cuban regime was not interested in antagonizing the Trump administration,” said Craig Deare, who was fired last February as the National Security Council’s senior Latin America specialist after he criticized Trump’s confrontational approach to Mexico. “It didn‘t make sense to me then and it doesn’t make sense to me now.”

The diplomats’ expulsions and the travel warning, along with the earlier tightening of the embargo and the hurricane, have already cut the flow of American tourists to the island. American business activity has dropped off further, in part due to the departure of Cuban diplomats in Washington who set up meetings and processed visas. Cuban dissidents also have complained that declining tourism has badly hurt small, independent businesses like guest houses, family restaurants and the like.

The Cuban government’s own investigation into the incidents has been another central piece of its public relations counteroffensive. According to Cuban news accounts, some 2,000 people have been involved in the inquiry, in which police detectives have questioned neighbors of the diplomats (who said they did not recall hearing anything unusual), Cuban doctors (who wondered why the Americans had never sought attention for their acute problems) and their own battery of scientists and technologists.

Cuban engineers also analyzed recordings that officials said were made by the American diplomats. The engineers also concluded that the noises were at decibel levels too low to cause hearing loss — but that the primary sounds on the recordings were made by cicadas. Other Cuban scientists have suggested that the Americans’ illnesses were psychosomatic.

Despite months of scrutiny by American intelligence assets, officials said U.S. intelligence agencies have gathered virtually no secondary evidence that Cuba might have assisted directly or indirectly in attacks on the Americans. Nor is there any indication that the Cuban government has identified some rogue faction of security forces that might have wanted to undermine the rapprochement with Washington, officials said.

The idea of such a rogue element working to subvert a major government initiative has been bandied about frequently in Washington in recent months. Although the inner workings of the Castro regime have always been somewhat opaque to outsiders, many longtime analysts of the Cuban politics are skeptical. “It’s hugely ironic that the rogue faction theory is coming from exactly the same people who say the Cuban government knows absolutely everything that’s going on in the country,” Armstrong, the former senior CIA analyst, said. “But there has never been any evidence of rogue factions working outside the system.” He recalled that in the one case that perhaps came closest — the show-trial conviction of several influential military and intelligence officers for drug trafficking and other crimes in 1989 — there was even some circumstantial evidence that the illicit activities had been tolerated by superiors.

Other than a few wildly far-fetched possibilities — North Korean agents running around Havana, or perhaps a secret team of Venezuelan spies subverting their own government’s closest ally — that would seem to leave only Russia. For Moscow, helping to derail the hard-won entente between Washington and Havana might constitute a geopolitical masterstroke, some U.S. officials said. It would fit into the Kremlin‘s aggressive campaign to undermine its western adversaries, using everything from espionage operations to election cyberattacks. Russia also has a long history of harassing American diplomats, a pattern that has intensified in Moscow since 2014, said Andrew Foxall, director of the Russia Studies Center at the Henry Jackson Society, a London think tank.

The U.S. embassy in Havana on Oct. 3, 2017. With only a skeleton staff remaining, Cuba experts say diplomats will have trouble monitoring a period of crucial political transition. (Yamil Lage/AFP/Getty Images)

After some years of Cuban hostility following the Soviet Union’s collapse and Russia’s withdrawal of the vast subsidies it had provided for decades, the Kremlin has made new efforts to solidify the two countries’ strategic bond. Russia has helped to offset the loss of Venezuelan oil imports with 1.9 million barrels of fuel (estimated to be worth $105 million at discounted rates), and Russian exports to Cuba nearly doubled last year. In December, Raúl Castro received the head of the Russian state energy giant Rosneft, stirring speculation that a major oil-exploration or supply deal might be in the works. The two countries’ security relationship has also grown. In December of 2016, just as the incidents affecting U.S. personnel began, Russia and Cuba signed a new agreement on defense and technology cooperation.

Along with a possible motive, the Russians might have the technological means — or at least the capacity to have plausibly developed a directed-energy weapon that U.S. scientists could not identify. Yet by now, officials said, intelligence analysts would also have expected to have culled from electronic intercepts of overseas conversations at least some secondary evidence that the Russians might be involved — suspicious telephone or email conversations, suggestive messages, movements of Russian agents — something. But officials said they have found virtually nothing that would constitute real evidence. They also wonder whether Russia would risk its growing relationship with Cuba by carrying out an operation that could undermine the island’s most important diplomatic initiative in decades.

Even if Russia had developed some new and compact directed-energy weapon that could have been used to attack the American diplomats, there would still have been extremely complex logistical challenges to its deployment. Russian agents would presumably have had to locate at least two dozen American diplomats in Havana, reach them covertly and repeatedly, and in some of the most heavily policed areas of what many consider a police state. Nor have intelligence agencies documented tests of a similar weapon on some other target, or signs that Russia might have moved agents into Cuba to carry out such an operation.

In the continuing absence of any real evidence of how the Americans were stricken, the Trump administration appears to have no easy path forward. About 10 of the diplomats and spouses continue to undergo vestibular and neurological rehabilitation, both in Washington and at the University of Pennsylvania’s Perelman School of Medicine. Some have moved on to new jobs in Washington or overseas, or have been kept busy in the Western Hemisphere Affairs bureau with such tasks as processing Freedom of Information Act requests or handling employment applications with the human resources staff, officials said.

By March 4, the State Department will have to decide whether to make the withdrawal of the diplomats a permanent reduction in staff. An internal department document obtained by ProPublica also suggests that the slowdown of consular activity may make it difficult for the United States to meet its commitment to processing at least 20,000 immigrant visas for Cubans this year, an annual target that is important to Cuban-Americans seeking to bring relatives from the island. American diplomats — including some of those forced to leave Havana — also say that the department has also reduced its ability to see, understand and perhaps influence what is happening in Cuba at a potentially historic transition point.

“Our diplomats want to go back,” one American official who has been extensively briefed on the developments in Havana said. “But if you can‘t get to the bottom of this situation, how does that happen?”