The victims reported waking up feeling unwell and hearing scratching sounds, as if something metal was being pulled across the floor or insects were scuttling about the room. Sometimes, those affected with later symptoms such as dizziness, hearing loss, and even concussions, reported hearing nothing at all.

In a response today, Secretary of State Rex Tillerson issued an order declaring all non-emergency personnel assigned to the U.S. Embassy in Havana, as well as their families, immediately return home until “the Government of Cuba can ensure the safety of our diplomats in Cuba.” Additionally, a travel warning is in effect and no further visas will be issued in Cuba while the staff is reduced some 60% to emergency personnel only.

While no device has been found that could be causing the illnesses to the 21 people who are believed to have been affected since the attacks started in November (just after the election of Donald Trump), CNN is reporting that the device is likely sonic in nature:

The devices used in the incidents have never been found, two US officials said, but appeared to be a type of sonic weapon that emitted sound waves capable of inflicting physical harm.

But the physical symptoms that people exhibited varied greatly, preventing doctors consulted in the United States from reaching a conclusion about what caused the trauma, two US officials said.

US government technical experts were also baffled. Some affected diplomats had lines of sight to the street in their homes, while others had shrubbery and walls that blocked views of their homes. Some heard loud sounds when the incidents took place, while others heard nothing.

It does not appear either the US Embassy or the ambassador’s residence were ever targeted, three senior US officials told CNN.

There may have been as many as 50 attacks, CNN reports; and, at least initially, Cuba was working with the U.S. to determine what was happening to diplomats while they slept. However, the U.S. has grown increasingly frustrated with the slow pace of the investigation and with the fact that Cuba, a near surveillance state, is continuing to insist it cannot pinpoint the culprit of the attacks.

“US officials said the Cubans’ denials strain credibility as the incidents took place in the diplomats homes and in hotels, locations that Cuba’s extensive intelligence apparatus are known to closely monitor,” CNN reported. “The officials said they believe even if the Cubans didn’t know about the incidents at the beginning of the investigation, they must have a clearer idea of what transpired than they are letting on.”

Tillerson, after urging from Republican Sens. Marco Rubio, R-Florida; Tom Cotton, R-Arkansas; Richard Burr, R-North Carolina; John Cornyn, R-Texas; and James Lankford, R-Oklahoma to close the US Embassy in Havana, appears to have decided to bring the diplomats home instead to prevent any further attacks.