Secretary of State Rex Tillerson announced Friday that America would withdraw all “non-emergency personnel” from the U.S. embassy in Cuba and all family members following months of unexplained attacks on American diplomats that have left some with hearing loss and, reportedly, brain damage.

“On September 29, the Department ordered the departure of non-emergency personnel assigned to the U.S. Embassy in Havana, as well as all family members,” Tillerson said in a statement. “Until the Government of Cuba can ensure the safety of our diplomats in Cuba, our Embassy will be reduced to emergency personnel in order to minimize the number of diplomats at risk of exposure to harm.”

Tillerson added that the department has issued a warning to all American citizens “to avoid travel to Cuba.” While emphasizing that those affected by the attacks in question have only been diplomats and residents, the statement noted that the locations where the attacks occurred are also “frequented by U.S. citizens.”

“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,” Tillerson said. Unlike remarks by spokeswoman Heather Nauert on the matter, Tillerson refers to them as “attacks;” Nauert told reporters Thursday she preferred the word “incidents.”

“We maintain diplomatic relations with Cuba, and our work in Cuba continues to be guided by the national security and foreign policy interests of the United States. Cuba has told us it will continue to investigate these attacks and we will continue to cooperate with them in this effort,” the statement concluded.

The announcement closely follows a meeting between Tillerson and his Cuban counterpart, Foreign Minister Bruno Rodríguez, who engaged in a conversation the State Department described as “firm and frank” in which Tillerson “conveyed the gravity of the situation and underscored the Cuban authorities’ obligations to protect Embassy staff and their families under the Vienna Convention,” according to the department’s statement.

The Cuban government has denied any involvement in the attacks and alleged it is conducting its own investigation into the incidents. At press time, Granma, the Cuban Communist Party newspaper, has not remarked on Tillerson’s statement.

The State Department, in the interest of the privacy of those affected, has not specified what types of attacks these have been or detailed the symptoms beyond what Tillerson described in his statement Friday. Reports in the Associated Press and other major media outlets suggest, however, that the victims appear to have been exposed to some sort of advanced sonic or acoustic weapon that causes significant hearing loss, headaches, concussion-like symptoms, and brain damage. The State Department has confirmed at least 21 cases, though some reports suggest the number may be higher.

While the United States has denied that it has evidence suggesting Cuban officials have conducted the attacks, there is evidence Cuba may possess the technology necessary for them. In remarks this year prior to the revelation that these attacks had occurred, a former Cuban political prisoner, Luis Zúñiga, told a human rights panel that he had seen Cuban officials use a sonic device to torture prisoners of conscience. One prisoner, he said, was found dead after guards placed large speakers near his cell that began emitting “shrill sounds” incessantly.

Cuban dissidents have repeatedly urged the Trump administration to shut down the U.S. embassy in Cuba and the Cuban embassy in Washington, arguing that legitimizing the Castro regime has significant negative repercussions for Cuban pro-democracy dissidents, who face state violence on a regular basis. The communist Castro regime has also historically used diplomatic stations to spy on unfriendly countries and harassing American diplomats in particular.

President Barack Obama reopened the U.S. embassy in Havana as part of a larger series of concessions to the Castro regime in 2014, intended to bring Washington closer to the rogue state.

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