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WASHINGTON — There must be an answer.

Whatever is harming U.S. diplomats in Havana, it has eluded the doctors, scientists and intelligence analysts scouring for answers. Investigators have chased many theories, including a sonic attack, electromagnetic weapon or flawed spying device.

Each explanation seems to fit parts of what's happened, conflicting with others.

The United States doesn't even know what to call it. Secretary of State Rex Tillerson used the phrase "health attacks." The State Department prefers "incidents."

Either way, suspicion has fallen on Cuba. But investigators also are examining whether a rogue faction of its security services, another country such as Russia, or some combination is to blame, more than a dozen U.S. officials familiar with the investigation told The Associated Press.

Those officials spoke on condition of anonymity because they weren't authorized to publicly discuss the investigation. The AP also talked to scientists, physicians, acoustics and weapons experts, and others about the theories being pursued.

Perhaps the biggest mystery is why the symptoms, sounds and sensations vary so dramatically from person to person.

Of the 21 medically confirmed U.S. victims, some have permanent hearing loss or concussions, while others suffered nausea, headaches and ear-ringing. Some are struggling with concentration or common word recall, the AP has reported. Some felt vibrations or heard loud sounds mysteriously audible in only parts of rooms, and others heard nothing.

"These are very nonspecific symptoms. That's why it's difficult to tell what's going on," said Dr. H. Jeffrey Kim, a specialist on ear disorders at MedStar Georgetown University Hospital, who isn't involved with the investigation.

To solve the puzzle, investigators are sorting symptoms into categories, such as auditory and neurological, according to individuals briefed on the probe.

There can be a lag before victims discover or report symptoms, some of which are hard to diagnose. So investigators are charting the timeline of reported incidents to identify "clusters" to help solve the when, where and how of the Havana whodunit.

While Cuba has been surprisingly cooperative , even inviting the FBI to fly down to Havana, it's not the same as an investigation with the U.S. government in full control.

"You're on foreign soil," said David Rubincam, a former FBI agent who served in Moscow. "The quality of the information and evidence you collect is limited to what the host government will allow you to see and hear and touch and do."

Especially when you don't even know what you're looking for.

Sonic device

The first signs pointed to a sonic attack. But what kind?

Some victims heard things — signs that the sounds were in the audible spectrum. Loud noise can harm hearing, especially high-decibel sounds that can trigger ear-ringing tinnitus, ruptured ear drums, even permanent hearing loss.

But others heard nothing, and still became ill. So investigators considered inaudible sound: infrasound, too low for humans to hear, and ultrasound, too high.

Infrasound often is experienced as vibration, like standing near a subwoofer. Some victims reported feeling vibrations.

And it's not impossible that infrasound could explain some of what diplomats thought they heard.

Though infrasound is usually inaudible, some people can detect it if the waves are powerful enough. For example, individuals living near infrasound-generating wind turbines have described pulsating hums that have left them dizzy, nauseous or with interrupted sleep. Such effects have prompted fierce scientific debate.

The balance problems reported in Havana? Possibly explained by infrasound, which may stimulate cells in the ear's vestibular system that controls balance, scientists say.

But there's little evidence infrasound can cause lasting damage once the sound stops.

And the pinpointed focus of the sound, reported by some? Infrasound waves travel everywhere, making them difficult to aim with precision.

"There's no efficient way to focus infrasound to make it into a usable weapon," said Mario Svirsky, an expert on ear disorders and neuroscience at New York University School of Medicine.

If not infrasound, maybe ultrasound?

At high-intensity, ultrasound can damage human tissue. That's why doctors use it to destroy uterine fibroids and some tumors.

But ultrasound damage requires close contact between the device and the body. "You cannot sense ultrasound from long distances," Svirsky said. No victim said they saw a weird contraption nearby.

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None of these sound waves seems to explain the concussions. Usually, those follow a blow to the head or proximity to something like a bomb blast.

"I know of no acoustic effect or device that could produce traumatic brain injury or concussion-like symptoms," said Juergen Altmann, an acoustic weapons expert and physicist at Germany's Technische Universitaet Dortmund.

Electromagnetic weapon

It may sound like Star Wars fantasy, but electromagnetic weapons have been around for years. They generally harm electronics, not humans.

The electromagnetic spectrum includes waves like the ones used by your cellphone, microwave and light bulbs.

And they can be easily pinpointed. Think lasers. Such waves can also travel through walls, so an electromagnetic attack could be plausibly concealed from afar.

There's precedent. For more than a decade ending in the 1970s, the former Soviet Union bombarded the U.S. Embassy in Moscow with microwaves. The exact purpose was never clear.

What about the sounds people heard?

Microwave pulses — short, intense blasts — can cause people to "hear" clicking sounds. According to a two-decade-old U.S. Air Force patent, the American military has researched whether those blasts could be manipulated to "beam" voices or other sounds to someone's head.

But when electromagnetic waves cause physical damage, it usually results from body tissue being heated. The diplomats in Cuba haven't been reporting burning sensations.

Something else

The stress and anxiety about the disturbing incidents could be complicating the situation. Diplomats may be taking a closer look at mild symptoms they'd otherwise ignored.

After all, once symptoms emerged, the U.S. Embassy encouraged employees to report anything suspicious. Many of these symptoms can be caused by a lot of different things.

At least one other country, France, tested embassy staffers after an employee reported symptoms. The French then ruled out sonic-induced damage, the AP reported .

Not knowing what's causing the crisis in Cuba has made it harder to find the culprit. If there is one at all.

The Cuba theory

It was only natural that American suspicion started with Cuba.

The attacks happened on Cuban soil. The two countries routinely harassed each other's diplomats over a half-century of enmity. Despite eased tensions over the past couple of years, distrust lingers.

Diplomats reported incidents in their homes and in hotels. Cuban authorities would know who is staying in each.

But what's the motive?

When symptoms emerged last November, Cuba was working feverishly with the U.S. to make progress on everything from internet access to immigration rules before President Barack Obama's term ended. Officials still don't understand why Havana would at the same time perpetrate attacks that could destroy its new relationship with Washington entirely.

Cuban President Raul Castro's reaction deepened investigators' skepticism, according to officials briefed on a rare, face-to-face discussion he had on the matter with America's top envoy in Havana.

Predictably, Castro denied responsibility. But U.S. officials were surprised that Castro seemed genuinely rattled, and that Cuba offered to let the FBI come investigate.

Then, Canadians got ill. Why them?

The warm, long-standing ties between Cuba and Canada made it seem even less logical that Castro's government was the culprit.

The rogues

If not Castro, could elements of Cuba's vast intelligence apparatus be to blame? Investigators haven't ruled out that possibility, several U.S. officials said.

It's no secret that some within Cuba's government are uneasy about Raul Castro's opening with Washington.

"It's entirely possible that hard-line elements acted," said Michael Parmly, who headed the U.S. mission in Havana until 2008.

But mounting unauthorized attacks, tantamount to aggression against a foreign power, would be a risky act of defiance in a country noted for its strong central control.

Cuba's surveillance of U.S. diplomats in Havana is intense. The government tracks U.S. diplomats' movements and conversations.

So at a minimum, if Americans were being attacked, it's difficult to imagine Cuba's spies being left in the dark.

The outsiders

Who else would dare?

U.S. investigators have focused on a small group of usual suspects: Russia, Iran, North Korea, China, Venezuela.

Russia, in particular, has harassed American diplomats aggressively in recent years.

Moscow even has a plausible motive: driving a wedge between the communist island and "the West" — nations such as the United States and Canada. Russia also has advanced, hard-to-detect weaponry that much of the world lacks and might not even know about.

None of the officials interviewed for this story pointed to any evidence, however, linking Russia to the illnesses. The same goes for the other countries.

Spying gone awry?

Maybe no one tried to hurt the Americans at all.

Several U.S. officials have emphasized the possibility the culprit merely surveilled the U.S. diplomats using some new, untested technology that caused unintended harm.

You might think eavesdropping devices simply receive signals. But the world of espionage is full of strange tales.

During the Cold War, the U.S. Embassy in Moscow discovered Russia listening to conversations through a wooden plaque that the American ambassador received as a gift. The plaque had a tiny "microphone" and antenna embedded, but no power source, making it hard to detect even when the room was swept for bugs.

The Russians had developed something novel. They remotely beamed electromagnetic waves to activate the device, which then transmitted sound back via radio frequencies.

Yet if the Cubans or anyone else were equally as innovative, it's unclear why the incidents would have continued once the United States and Canada complained.

Associated Press writers Michael Weissenstein in Havana, Ben Fox in Miami, and Matthew Lee, Bradley Klapper and Sadie Gurman in Washington contributed.