The United States has issued a travel warning against Cuba, advising Americans not to visit and removing some personnel from the island in the wake of what are believed to be sonic attacks on diplomats.

Here’s what to know about what’s going on in Cuba.

Who was attacked in Cuba?

21 U.S. diplomats and family members have been experiencing mysterious health problems since last November. The incidents have caused a range of symptoms including “ear complaints and hearing loss, dizziness, headache, fatigue, cognitive issues, and difficulty sleeping,” according to the State Department.

Who is behind the Cuba attacks?

The U.S. government hasn’t determined who is behind the attacks, and the Cuban government denies any involvement. The U.S. government is now describing the situation as “specific attacks” on American diplomats, which some believe could have been caused by a sonic device.

What is a sonic attack?

No one has yet discovered a device or piece of equipment in connection with the attacks in Cuba. But, according to ABC News, exposing people to sound waves above and below the range of human hearing could cause serious damage, affecting different people in different ways. “The Department does not have definitive answers on the cause or source of the attacks and is unable to recommend a means to mitigate exposure,” Secretary of State Rex Tillerson said in a statement Friday.

Who is affected by the Cuba travel warning?

About 60% of U.S. staff in the embassy in Havana will be evacuated, according to the Associated Press, and the embassy will stop processing visas in Cuba indefinitely. The State Department is telling American tourists against visiting Cuba: “Because our personnel’s safety is at risk, and we are unable to identify the source of the attacks, we believe U.S. citizens may also be at risk and warn them not to travel to Cuba,” the official warning states. For now, Cuban diplomats are not being asked to leave Washington.

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Write to Tessa Berenson at tessa.berenson@time.com.