It’s long been known that working in a loud environment can have adverse effects on your hearing. In fact, any workplace where noise is consistently above 85 decibels can cause permanent hearing loss. But now, workers at the U.S. Embassy in Cuba have also been reporting hearing problems and other mysterious injuries. The cause?

Officials believe espionage is to blame.

It started when the U.S. Embassy was reopened in Cuba in 2015 under Barack Obama. Almost immediately, diplomats began experiencing hearing loss and a wide range of other symptoms. In some cases, the symptoms were so severe that diplomats were forced to cancel their tours and return to the United States.

So far, little information has come out publicly. The most likely theory, however, seems to be that the diplomats were subjected to a sonic listening device used for covert campaigns. Experts believe the “sonic weapon” operated outside the range of audible sound perceivable by the human ear. Despite this, some victims may have traumatic brain injuries.

A State Department spokeswoman was quick to try and silence speculation, saying that “We don’t have any definitive answers about the source or the cause of what we consider to be incidents.”

Since then, diplomats from other nations have reported similar symptoms. Global Affairs Canada spokeswoman Brianne Maxwell confirmed the involvement of a Canadian diplomat in the case, reporting that “[we] are aware of unusual symptoms affecting Canadian and US diplomatic personnel and their families in Havana.”

So far, the Cuban government denies any involvement or responsibility for the actions that have lead to the hearing loss. They have since launched their own investigation.

“Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families,” reads a statement from the Cuban foreign ministry, as reported by NBC News. “It reiterates its willingness to cooperate in the clarification of this situation.”

While it is likely that the U.S. diplomats were being closely watched by the Cuban state security apparatus, officials report that they are looking into a number of possible scenarios that could have been carried out without Cuban involvement, including an intelligence campaign by Russia.

The one silver lining to this story is that it is unlikely that the sonic device was meant to harm any diplomats. According to the Washington Post, any such attack would be unprecedented.