FILE PHOTO: People walk past the U.S. Embassy in Havana, Cuba, June 19, 2017. REUTERS/Alexandre Meneghini

WASHINGTON (Reuters) - U.S. officials are aggressively investigating what caused symptoms ranging from hearing loss to mild brain injury in 21 people linked to the U.S. Embassy in Cuba, a State Department official said on Thursday.

Spokeswoman Heather Nauert told a briefing that no new incidents had been reported since late August but testing continued, with 21 people saying they had been injured in what some media accounts have called an “acoustic attack.”

“We are, at the State Department, very deeply concerned about what has taken place and what has happened to our American personnel who have been serving at our embassy in Cuba,” Nauert said. “This has certainly turned out to be a difficult situation for some of our people.”

The incidents, which began in late 2016, were first confirmed by a U.S. official in August. Nauert said at that time 16 people had been injured.

Some of those had returned home for testing and treatment, Nauert said, while others had been tested in Cuba, where the embassy has a fulltime medical officer.

News media have reported that Americans and Canadians working in Cuba have been diagnosed with hearing loss, nausea, headaches, balance disorders and conditions as serious as mild traumatic brain injury and damage to the central nervous system.

“The investigation into all of this is still underway. It is an aggressive investigation that continues and we will continue doing this until we find out who or what is responsible for this,” Nauert said.

She said U.S. diplomatic security personnel had been able to look through the rooms of some of those injured and conducted searches but still had not determined the cause of the injuries.

Nauert said people were still being tested and it was possible the number of injured personnel would rise.