More than half of the staff at the American Embassy in Havana, Cuba will return to the US in the wake of mysterious attacks that left diplomats with hearing damage and brain injuries, the State Department announced Friday.

The department said that the staff drawdown was necessary to ensure their safety. And “because our personnel's safety is at risk, and we are unable to identify the source of the attacks, we believe US citizens may also be at risk and warn them not to travel to Cuba,” the department added in a travel advisory.

The attacks have injured 21 Americans associated with the embassy so far, as well as Canadian diplomats. Attacks took place in the diplomats' homes as well as hotels. The last attack occurred in August, according to the New York Times

Victims have reported a range of symptoms, including dizziness, nausea, headaches, balance problems, ringing in the ears (tinnitus), nosebleeds, difficulty concentrating and recalling words, permanent hearing loss, and speech problems. There have also been brain injuries, including swelling and concussion.

Victims also reported bizarre circumstances under which the attacks occurred, including feeling vibrations and hearing deafening noises only while lying in bed. Others said they didn’t notice anything unusual before their symptoms developed.

Authorities are baffled as to the cause of the injuries. The FBI carried out an extensive investigation of rooms and surveillance footage, but they found nothing. Some have speculated that faulty surveillance equipment or some sort of sonic weapon could be involved. But as Ars reported previously, scientists say that no single instrument could explain all the symptoms reported. Specifically, a sonic weapon would not cause brain damage.

“Brain damage and concussions, it’s not possible,” Joseph Pompei, a former MIT researcher and psychoacoustics expert told the AP earlier this month. “Somebody would have to submerge their head into a pool lined with very powerful ultrasound transducers.”

Equally unclear is who is behind the attacks. US authorities do not suspect that the Cuban government is behind them because officials there have been forthcoming and fully cooperative with the investigations. And the fact that a Canadian diplomat was attacked also drew suspicion away from the Cuban government; the two countries have a long-standing friendly relationship.

“Whoever is doing this obviously is trying to disrupt the normalization process between the United States and Cuba," Leahy told the Times today. “Someone or some government is trying to reverse that process.”

Authorities speculated that a third government, such as Russia, or a rogue government unit may be to blame.

With so many questions swirling, Secretary of State Rex W. Tillerson met with Cuba’s foreign minister Bruno Eduardo Rodríguez Parrilla in Washington a few days ago. The meeting was at the Cuban government's request. But it failed to convince Tillerson that Cuba could keep Americans safe, thus prompting today’s announcement of the staff pullback and a travel warning.

The Embassy will remain open, but only emergency staff will remain to perform essential services, such as aiding American travelers.