Some Americans based at the U.S. Embassy in Havana, pictured above, reported nausea, hearing loss, headaches and balance problems.

At least 16 Americans associated with the U.S. Embassy in Cuba suffered damage such as hearing loss while serving in Havana, the State Department said Thursday.

The figure is the first accounting of the scope of what the U.S. government suspects was a deliberate targeting of U.S. diplomats. Secretary of State Rex Tillerson has confirmed that Americans serving in Havana began having symptoms that were at first unexplained but were later found to be related.

The Americans based in Cuba started reporting having problems late last year, at the end of the Obama administration, and the problems have continued into the first months of the Trump administration. Initially, five Americans were reported to have been injured, as well as at least one Canadian diplomat.

The cause, however, remains a mystery.

The State Department suspects that they may have fallen victim to some sort of “acoustic attack,” though the exact origin is still being investigated by the State Department and the FBI. The Cuban government, which has denied having anything to do with targeting diplomats, is cooperating with the U.S. investigation, officials said.

CBS News reported this week that it had examined medical records of some of the affected diplomats. Doctors examining them diagnosed brain injuries that led to complaints of nausea, hearing loss, headaches and balance problems.

State Department spokeswoman Heather Nauert said Thursday that the affected “U.S. government employees, members of our embassy community” in Havana, had been treated by physicians in the United States and in Cuba.

It is unclear whether the Americans who were harmed are all State Department employees or whether the 16 include family members.

In May, the United States expelled two Cuban diplomats in response to the incidents that apparently happened, among other places, in the Cuban government housing provided to foreign diplomats.

Although the State Department has not accused the Cuban government of being directly responsible for harming Americans, it says Havana failed its obligations under the Vienna Convention to keep foreign diplomats safe.

Harassment of U.S. diplomats in Cuba used to be routine. Their homes were broken into and messed up, and they were regularly tailed when they went out. But reports of diplomats being physically harmed were rare.

U.S. officials who worked in Havana said the petty harassment has slacked off in recent years.