An acoustic attack on U.S. and Canadian diplomats in Cuba is “unprecedented,” State Department Spokeswoman Heather Nauert said Wednesday, after revelations that American employees have suffered severe health problems including mild brain injury and damage to the central nervous system believed to have come from exposure inaudible high-pitch sound.

“This is unprecedented, we have not seen this type of activity take place before,” Ms. Nauert said. “Nowhere.”

The occurrence of a sonic attack on North American diplomats first came to light earlier this month with the State Department saying staff at the embassy in Havana experienced “physical symptoms,” and the Associated Press reporting confirmation by U.S. Secretary of State Rex Tillerson that some staff suffered “hearing loss.”

Other U.S. employees were reported to have had headaches, nausea and balance disorders, among other symptoms. The State Department would not comment on the symptoms reported but said the U.S. government had sent medical professionals to embassy staff in Havana in addition to evacuating more serious cases to a Miami Hospital.

“Like any top-notch academic medical center in the nation, the University of Miami is often consulted regarding complex health care issues or emerging diseases. In the case of U.S. diplomats, our physicians were consulted by the State Department,” a statement from the university said, reported CBS.

A spokeswoman for Johns Hopkins confirmed that one of their doctors is advising the State Department on the issue.

Earlier this month it came to light that since December 2016, U.S. staff in Havana had been complaining of a range of symptoms that included nausea, problems with balance and hearing loss. Some staff were evacuated to medical facilities in Miami, Florida for treatment but others were treated in Havana by U.S. medical personnel brought in by the U.S. government, Ms. Nauert said.

“This is a matter we take very seriously, we are working and have been working to provide our staff and U.S. government employees with the best medical attention that we can provide to them.”

An investigation is ongoing across multiple U.S. agencies, Ms. Nauert added.

CBS news reported that a source within the State Department said it’s possible the attacks occurred either in diplomatic housing — provided by the Cuban government — or close by.

Earlier this month, the Cuban government denied any involvement in the attacks in a statement to CBS: “Cuba has never permitted, nor will permit, that Cuban territory be used for any action against accredited diplomatic officials or their families, with no exception.”

Stopping short of calling it reciprocity for the attacks, Ms. Nauert confirmed in early August that two employees in the Cuban Embassy in the U.S. were asked to return to their country.