So-called “sonic attacks” can result in hearing loss or brain damage and are created by “long-range acoustic devices” that are often used over the open water, where they can be unleashed across greater distances without losing volume.

At the U.S. Embassy in Havana, Cuba, State Department employees first came under attack in the fall of 2016, a pattern that continued through to April.

On Friday, the State Department announced that it would remove 60 percent of its staff from the embassy in Havana after the mysterious sonic attacks caused health problems for 21 people. The problems have been frightening and severe: Workers at the embassy affected by the sonic attacks suffered mysterious hearing loss. CBS News reported that “the attacks were directed at their homes, which the Cuban government provides. The last reported incident was in August.”

State Department spokesperson Heather Nauert said in August that “the incidents are not ongoing at this point.”

The U.S. government hasn’t found a cause of the sonic attacks. Snopes points out that the health problems could be a result of a variety of things, and argues that “ultrasonic devices” are unlikely:

…an ultrasonic device could be responsible for the transient occurrence of fatigue, headaches, ringing ears, vertigo, and nausea and (even more dubious) concentration and memory issues. Taking a similarly loose approach for infrasound, this kind of noise could cause hearing loss, nausea, and (most dubiously) some form of organ discomfort or damage. Neither can explain all of the symptoms.

“Nothing has been identified as, ‘Here is a piece of equipment,’” Nauert told reporters. “We’re not assigning responsibility at this point. We don’t know who the perpetrator was of these incidents.

The State Department will also warn Americans to not travel to Cuba, and it suspended the issuance of any visas in Havana, reported El Nuevo Herald.

The FBI is investigating the mysterious attacks. The New York Times observed these mysterious attacks seem to have shaken the Cuban government as well:

That the Cubans offered to let the F.B.I. go to Havana and investigate represented a rare level of openness and was seen as yet another indicator that the Cubans themselves have been shaken by the incident.

Only essential personnel will remain at the American Embassy in Havana. The building that houses the embassy was largely closed due to conflict with the Cuban government for much of the second half of the last century but reopened on August 14, 2015.

Theories About the “Moscow Signal”

A number of theories have been posited online for the cause of the sonic attacks. Wikileaks found a similarity to the “Moscow Signal,” a tactic used by the former Soviet Union during the Cold War in the early ‘50s.

As described in a 1979 issue of Science, the Moscow Signal was defined as a microwave beam that was pointed at the American Embassy in Moscow, first reported by investigative journalist Jack Anderson in 1972.