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Aug. 9 (UPI) — Two Cuban diplomats in the United States were expelled in May after suspicions that American officials in Havana were hit with a silent “acoustic attack” that induced hearing loss, State Department officials said.

“Some United States government personnel who were working at our embassy in Havana, Cuba, on official duties have reported some incidents which have caused a variety of physical symptoms,” State Department spokeswoman Heather Nauert said Wednesday during a press briefing. “We don’t have any definitive answers about the source or the cause of what we consider to be incidents.

State Department officials told CNN that one American employee in Havana may have permanent hearing loss from the alleged attacks in 2016. Several others experienced physical symptoms that resembled concussions while in Cuba.

The cause of the symptoms and method of attack is unknown.

“It can be quite serious,” a State Department official said. “We have worked with the Cubans to try and find out what is going on. They insist they don’t know, but it has been very worrying and troublesome.”

The allegations of Cuba using sonic weapons to induce physical harm on American employees in Havana comes at a time of tension between the two countries. After former President Barack Obama loosened restrictions on travel to the island nation, President Donald Trump has threatened to roll them back and enforce tighter travel and trade restrictions.

“The Cuban government has a responsibility and an obligation under the Geneva convention to protect our diplomats,” Nauert said. “Do that is part of the reason why this is such a major concern of ours.”