The U.S. government is investigating whether American diplomats serving in Cuba were victims of an attack that damaged their hearing and caused other physical symptoms, officials said Thursday.

The Associated Press reported that officials concluded the Americans were targeted by a “covert sonic weapon.” The AP described the suspected weapon or device as something that could not be heard by the victims.

A U.S. official confirmed the diplomats suffered hearing damage and neurological symptoms that remain unexplained.

The official did not identify a covert weapon as the suspected cause but said ongoing FBI and State Department investigations are looking at the incidents as an attack. The official spoke on the condition of anonymity because of a lack of authorization to discuss the investigations on the record.

Another official said the incidents occurred at diplomats’ homes. U.S. diplomats in Cuba live in compounds owned by the Cuban government.

The State Department had said Wednesday that two Cuban diplomats were expelled from the United States in response to unexplained physical symptoms suffered by Americans stationed in Havana but declined to specify what kind of ailments were reported.

Department spokeswoman Heather Nauert would not confirm that the symptoms included hearing loss or damage or that a weapon may have been used.

“We will not confirm the health status of any Americans,” she said Thursday. “What I can tell you is that these were U.S. government personnel who were in Havana, in Cuba, on official duty,” she said.

Cuba failed in its obligation under the Vienna Convention to keep foreign diplomats safe, and the U.S. order to expel two Cubans was in response to that, she said. She would not directly blame Cuba for causing harm to the Americans.

“We consider these to be incidents because we are still trying to determine the actual cause of their situation. They have had a variety of physical symptoms, and that’s as far as I can go,” Nauert said.

When it comes to sound, the human body can be very fragile outside a narrow range. Certain low frequencies can lead to balance problems, soft-tissue pain and disturbed visual function. High ones can produce a powerful shock wave that can physically damage the ear.

However, Seth Horowitz, a former professor of neuroscience at Brown University, said that “there are no acoustic devices that can cause sudden onset hearing loss that the people involved could not hear,” and he expressed skepticism about the State Department’s claims.

Horowitz explained that infrasonic machines at a very high amplitude can cause nausea, ultrasonics have no effect on hearing, and long-range acoustic devices or sound canons are “extremely noticeable and easy to move away from.”

“It’s tragic,” he said of the reports, “but it can happen from viruses, genetic predisposition, an accident, exposure to urban noise.”

Michael Hoa, an ear surgeon at Georgetown University Hospital, said studies show that loud sounds can kill the ear’s “hair cells,” which are responsible for changing sound into an electrical signal to your brain. Kill enough of them, and your ability to hear is diminished. Although there is less known about the effect of sounds that cannot be heard, there’s evidence that they, too, can change the way the hair cells function.

“We clearly know that really loud sound can cause this kind of damage. It’s not inconceivable that other kinds of sound we can’t hear could cause damage in ways we haven’t thought of or looked at yet,” Hoa said.

Symptoms reported by U.S. personnel date to late 2016 and are not life-threatening, the State Department said. Some victims left Havana to receive medical treatment.

Cuba has denied involvement.

“Cuba has never permitted, nor will permit, that Cuban territory be used for any action against accredited diplomatic officials or their families, with no exception,” a statement from the Cuban Foreign Ministry said.

The Cuban government was informed of the incidents in February, the statement said. The expulsion of the two Cubans was “baseless,” the statement said.

Canada’s foreign ministry said Thursday that one Canadian diplomat suffered similar symptoms while serving in Havana in recent months.

Nauert would not comment on the Canadian statement.

Harassment of American diplomats has been common for decades, but physical attacks of any kind are exceedingly rare. American government officials are told to expect constant surveillance when serving in Cuba.

The U.S. Embassy opened following a 2014 diplomatic overture from President Barack Obama. Americans previously served at a diplomatic office that did not carry full embassy status, and Cubans did the same in Washington.

Washington Post writer Ariana Cha contributed to this report.