Starting in mid-November 2016, at least 16 American diplomats in Cuba started to feel ill. They complained of hearing loss, nausea, loss of balance, and headaches. An American doctor traveled to Havana in the spring to evaluate them, and other doctors have since reviewed their cases.

After months of investigation, US officials have a theory for what happened: The diplomats were attacked with some sort of secret Cuban sonic weapon. Making a strange and scary story even stranger and scarier, the Americans, along with one Canadian diplomat, were hit while inside the homes where they lived with their spouses and children — homes that had been provided by the Cuban government itself.

Today, some of the diplomats suffer from mild brain damage and blood disorders, and two may have completely and permanently lost their hearing. A few of them had to return to the US because of their injuries. And while the attacks appear to have stopped last spring, questions remain about why they occurred in the first place. Experts — including current and former officials who are intimately familiar with US-Cuba relations — are shocked and confused by the events.

“The unique nature of the incidents has been a complicating factor in answering definitively many of the questions that have arisen,” a State Department spokesperson told me. “We do not know who or what is causing these incidents.”

The probe is still underway to figure out the cause of what Secretary of State Rex Tillerson called “health attacks.” The Cuban government denies any involvement in the supposed attack.

“I have never heard of anything like this, and it’s a little hard to understand,” a senior US diplomat told me. “Why would this happen now?”

Here’s one possibility. After 50 years of tensions, US-Cuba relations improved dramatically under former President Barack Obama. The Cuban government knew that Donald Trump opposed Obama’s overtures and was far more hostile to their government, and they may have been using the sonic devices to try to listen in on American diplomats as part of an effort to learn more about the new administration. The injuries, in other words, could have come from a Cuban espionage operation gone awry.

Cuba has a history of harassing US diplomats. But not like this.

For years, American diplomats traveling to Cuba expected — and received — harassment from local spies.

“It was pretty low-tech and juvenile,” the senior US diplomat said in an interview. “We were always surveilled and followed. Our people would always know they got into our homes because they moved furniture and turned books around to make sure we knew they were there.”

“They would even leave feces on people’s toothbrushes and in people’s drinking glasses,” the diplomat continued.

Experts and former officials I spoke with detailed other incidents. Cuban spies would empty water tanks diplomats kept at their homes in case the water shut off in Havana, which happened frequently. They also partially blinded American drivers by turning car lights on very brightly behind them. But in these cases, Cuban officials were trying to inconvenience American diplomats — not hurt them.

That all happened during a low point in US-Cuba relations. For nearly five decades, the US government maintained a strict embargo on Cuba because of the Castro government’s alliance with the Soviet Union during the Cold War.

That began to change in 2013, when the Obama administration, with the help of the Vatican, opened secret talks with Havana. During that time, but especially by the end of 2014, Havana spies effectively stopped harassing American diplomats. That’s because Cuba didn’t want to jeopardize the negotiations. “The Cubans did not want to create an incident with us while talks began,” John Caulfield, chief of mission at the US Interests Section in Havana from 2011 to 2014, told me. “It would’ve been quite easy for an incident to send things back to zero.”

In 2014, Obama announced he wanted to open a US embassy in Cuba and increase travel and commerce with the country. He got his wish: The Cuban Embassy in Washington, DC, opened on July 20, 2015 — formally reestablishing ties between the two countries — and the US opened its embassy in Havana on August 14, 2015. Obama also lifted some travel restrictions to Cuba, allowing Americans to more easily visit the country.

But then Donald Trump won the election. Throughout the campaign, he promised to reverse the deal Obama made with Havana. Cubans legitimately wondered whether some of the improvements they’d seen — like more foreign hotels opening on the island — would go away. Cuban spies, then, may have taken extra measures to find out what Trump would do once in office. They had every incentive to want to keep close tabs on Trump’s plans.

To do that, Cuba may have turned to a sonic weapon. But it appears something went horribly wrong.

There are many theories about what happened, including the sonic weapon

One thing is for sure: Americans were under surveillance while the supposed attacks took place.

“Even if relations were getting better, Cubans are still obsessively interested in the activities of US diplomats in Cuba, and the fact that relations had improved did not remove those concerns,” Caulfield told me.

And they were definitely interested in Trump. Recall the timing: The attacks started in mid-November 2016 and ended around this spring. That coincides very neatly with Trump’s election and the first months of his administration. It’s possible Cuba turned to this weapon specifically to listen in on his transition plans and learn what they could about the new White House.

A sonic device used for spying would be hard to hear. That’s because they usually operate at the ultrasonic level — above audible range — or the infrasonic level, below audible range. Some doctors believe infrasound was more likely used in an attack like this because it travels farther than ultrasound, which means Cuban spies could use it from farther away.

However, CNN reports that some attacks were audible. Two US officials said the sonic attacks made a “deafeningly loud sound similar to the buzzing created by insects or metal scraping across a floor.”

Infrasound can cause some of the psychological effects the American and Canadian diplomats experienced. For example, a November 2001 study by the National Institutes of Health noted that one of the symptoms of exposure to infrasound is hearing loss.

It’s also possible that Cuba wasn’t behind this at all. It could’ve been another country that wanted to harm Americans or drive a wedge between the US and Cuba. But because Cuban spies surveil American officials so closely, Caulfield believes they would’ve known what was happening. Actions by another country “would’ve been immediately obvious to the Cubans,” he told me.

Another theory is that some elements of the Cuban intelligence services want US-Cuba relations to stay frosty. Attacking American diplomats would be a good way to do that. William LeoGrande, a Cuba expert at American University, told me that Cuba’s closer ties with the US led to some greater economic and political freedoms on the island. That’s made some Cuban officials fear they are losing grip on power, especially as the country slowly makes market-based reforms.

And while the why for what happened remains unclear, the US knew American diplomats got hurt. That was enough for the US to take action.

In May, America expelled two Cuban foreign service officers in retaliation for Havana’s failure to protect the US diplomats. The expulsions were done in secret and just became public this month.

“We requested their departure as a reciprocal measure,” Heather Nauert, the State Department’s top spokesperson, told reporters in August. “Under the Vienna Convention, Cuba has an obligation to take measures to protect diplomats.”

That provision says a host country must ensure for the safety of foreign diplomats.

Although action was taken, blame has yet to be assigned. Did Cuba use a sonic weapon or not? And if a device was used, was it meant to physically harm US and Canadian diplomats? Was it a spying tool that failed, or was it something else? It’s still unclear — but surely the diplomats and their families want answers.

In the meantime, Caulfield doesn’t expect something like this to happen again. “If it does, it would be a much more serious event because [the Cubans] would know the impact of using this technology,” he told me.