Secretary of State Rex Tillerson said Sunday that the United States is considering closing the U.S. Embassy in Havana in response to mysterious hearing problems that have left at least 21 employees with serious health issues.

“We have it under evaluation,” Tillerson said on CBS’s “Face the Nation” when asked about calls by some senators to shutter the diplomatic mission. “It’s a very serious issue, with respect to the harm that certain individuals have suffered, and we’ve brought some of those people home. It’s under review.”

Closing the embassy would be a serious setback to relations between the United States and Cuba, two Cold War adversaries whose enmity stretched more than half a century before they restored diplomatic relations and upgraded their missions into embassies in 2015.

But at least 21 Americans who worked in the U.S. Embassy in Cuba have reported medical problems since late last year, when percussive attacks on their residences began. The incidents apparently continued into 2017. Two Cuban diplomats have been expelled from the embassy in Washington in response.

The State Department did not talk publicly about the incident until August, months after the problems were uncovered.

The FBI is investigating what the union representing Foreign Service officers calls “sonic harassment attacks” on the diplomats at the U.S. Embassy in Havana. (Desmond Boylan/AP)

Some of the victims suffered mild traumatic brain injuries, hearing loss and other neurological and physical ailments, said the union representing Foreign Service officers. The FBI is investigating what the union calls “sonic harassment attacks” on the diplomats. A Canadian diplomat also reported similar problems.

Cuba has denied any responsibility for the attacks.

Cuban President Raúl Castro called in the then-head of the U.S. mission, Jeffrey DeLaurentis, to express concern.

Five Republican senators wrote Tillerson last week asking him to close the embassy and expel Cuba’s diplomats from the United States.

“We ask that you immediately declare all accredited Cuban diplomats in the United States persona non grata and, if Cuba does not take tangible action, close the U.S. Embassy in Havana,” the senators wrote. “Cuba’s neglect of its duty to protect our diplomats and their families cannot go unchallenged.”