U.S. diplomats were diagnosed with mild traumatic brain injuries and possible damage to the central nervous system from exposure to an “acoustic attack” in Cuba last year, CBS News reported Wednesday, citing obtained medical records.

The records suggest that the apparent attack caused more extensive damage than previously reported. The diplomats stationed in Havana reported symptoms that resembled concussion and hearing loss.

Following months of investigation, U.S. officials concluded that the diplomats had been exposed to an advanced device that operated outside the range of audible sound and had been deployed as sonar attacks either inside or outside their residences, Fox News previously reported. It was not immediately clear if the device was a weapon used in a deliberate attack, or had some other purpose.

Some of the diplomats’ symptoms were so severe they were forced to cancel their tours early and return to the United States, officials said.

“We hold the Cuban authorities responsible for finding out who is carrying out these health attacks,” Secretary of State Rex Tillerson said earlier this month.

Cuba’s Ministry of Foreign Affairs released a statement August 9 denying allegations.

“Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families, without exception,” the ministry said. “Moreover, it reiterates its willingness to cooperate in the clarification of this situation.”

Diplomats also reported to CBS News other forms of harassment including vehicle vandalization, constant surveillance, and home break-ins.