The great mystery behind what hurt two dozen U.S. diplomatic personnel in Cuba remains unsolved, according to a Senate hearing on Tuesday. But an FBI investigation is casting doubt on a once-popular theory: that embassy staff were the victims of "sonic attacks."

According to a report seen by the Associated Press, an FBI investigation has turned up no evidence that sound waves harmed American diplomats in Havana.

The report has not been released publicly. On Saturday, Arizona Republican Sen. Jeff Flake confirmed to AP that the FBI had told him about the lack of evidence supporting a sonic attack theory.

Many questions surrounding the incidents remain unanswered, more than two years since symptoms were first spotted.

From December 2016 to January 2017, U.S. personnel in Cuba sought medical treatment for various symptoms, including headaches, ear pain, dizziness and hearing problems, according to Dr. Charles Rosenfarb, the State Department's medical director.

"They associated the onset of these symptoms to their exposures with unusual sounds or auditory sensations. Various descriptions were given: 'a high-pitched beam of sound'; an 'incapacitating sound'; a 'baffling sensation' akin to driving with the windows partially open in a car; or just an intense pressure in one ear," Rosenfarb told the Senate Foreign Relations Subcommittee on the Western Hemisphere on Tuesday.

As news of those symptoms emerged last year, hypotheses swirled. They ranged from the use of an electromagnetic weapon to a sonic device targeting personnel with sound waves that were either super-loud or inaudible.

The incident led to the sharpest rise in tensions between the U.S. and Cuba since President Obama restored diplomatic relations with the island in late 2014.

The Trump administration pulled staff from Havana, expelled Cuban diplomats and issued a Cuba travel warning.

Cuban officials denied any involvement in harming U.S. personnel and have called the sonic attack allegations "science fiction." Canada is the only other country known to have complained that its diplomats were also affected.

Washington says Cuba should be doing more to protect diplomatic personnel.

"Cuba is a security state. The Cuban government in general has a very tight lid on everything and anything that happens in that country," acting Assistant Secretary of State Francisco Palmieri said at Tuesday's hearing.

A total of 24 Americans, including at least one relative of embassy personnel, reported the health problems. "Ten of the 24 patients have returned to either full- or part-time work, while others continue to receive treatment with an anticipation of return to duty," Rosenfarb said.

Secretary of State Rex Tillerson said in an AP interview he will not return U.S. diplomats to Cuba for now, and he is not convinced the alleged attacks are over. The State Department didn't divulge diplomatic staff numbers, but in 2017, Tillerson reportedly ordered more than half its 50 U.S. staff to leave the Havana embassy.

In addition to ongoing federal investigations, Palmieri said the State Department will set up an Accountability Review Board, which, as The Hill explains, is a special panel convened when a diplomat sustains serious injury abroad.

Also testifying before the Senate subcommittee, the State Department's diplomatic security director Todd Brown said investigators are not ruling out various possibilities, including a "viral" attack.

"There's viral, there's ultrasound — there's a range of things that the technical experts are looking at as, could this be a possibility?" Brown said.

Tom Udall, a Democratic senator from New Mexico, pressed him further: "When you say viral, you are talking about someone intentionally implanting a virus?" he asked.

"That would not be ruled out," Brown said, "it could be a possibility."

One potential factor for the health problems the officials said they have ruled out? "Mass hysteria."

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