The attacks were felt by at least five diplomats and their families, a larger number than first reported, leading officials to reconsider the idea that the Canadians were collateral damage, as the attacks affected more Americans. It is thought only Canadians and Americans experienced the attacks.

The victims – the guessing is that it was some kind of technology attack in their Havana homes – suffered hearing loss, headaches and dizziness.

All of the Canadians have recovered, but some of the 21 Americans affected suffered permanent hearing loss and mild brain damage, according to the U.S. State Department, the Canadian Broadcasting Corporation (CBC) reported.

The first complaints by U.S. diplomats came as early as last year, when some felt a vibration in their bodies while in certain parts of their homes. The U.S. complained to Cuba on Feb. 17. The attacks stopped, then started again.

Then Canadian diplomats experienced symptoms between March and May.

The Associated Press reported in August that after an investigation, U.S. officials concluded the diplomats were targeted by a device emitting a sound not audible to the human ear, that it was possibly some electronic surveillance device.

Cuba denied any involvement and President Raul Castro announced that the FBI would be welcome if it wanted to investigate.

The FBI took up the offer and went to Havana, where several homes were tested for electronic surveillance devices. The Royal Canadian Mounted Police went too to check Canadian domiciles. But neither intelligence agency uncovered any sonic devices.

The last attack occurred against the U.S. in Havana on Aug. 21, and officials are mystified even as the investigation continues.

“The reality is, we don’t know who or what has caused this,” U.S. State Department spokesperson Heather Nauert told the AP. “And that’s why the investigation is underway.”

Anadolu Agency