At least 16 Americans associated with the U.S. Embassy in Havana suffered symptoms from attacks on their health in Cuba that have still not been explained, the United States said Thursday.

Until now, the U.S. had not disclosed the number of Americans believed to be affected by the attacks, which caused a variety of symptoms including unexplained loss of hearing. State Department spokeswoman Heather Nauert said the “incidents” are no longer occurring. U.S. officials have said the symptoms started in the fall of 2016 and were last reported in April.

Earlier, U.S. officials had said the symptoms appeared to have resulted from a covert sonic device. But Nauert said no device has been found so far. The U.S. still hasn’t identified the perpetrator, and Nauert said that Cuba is cooperating with the U.S. investigation.

“We take this situation extremely seriously,” Nauert said.

The Americans affected include diplomats posted to the embassy and spouses. Nauert said that all have been treated by doctors either in Cuba or the U.S. Some of the 16 are still in Cuba, while others have returned to the U.S.

Canada’s government has said at least one Canadian diplomat in Cuba also had been treated for hearing loss.

(AP)