More information has come to light regarding the mysterious attacks perpetrated against American diplomats working at the U.S. Embassy in Cuba: 21 individuals have now been affected by what appears to be a series of attacks employing a covert sonic weapon. U.S. Secretary of State Rex Tillerson announced that they are considering closing the embassy in response to the incidents, a move that could severely damage recently renewed diplomatic ties between Cuba and the U.S.

Along with the number of individuals affected having risen to 21, the range of symptoms has also increased, including permanent hearing loss, headaches, nausea, and tinnitus. A number of those affected are also showing signs of brain swelling and concussions, and are having difficulty with mental concentration and common word recall.

One victim reported, according to Associated Press, that he was hit late at night with an "agonizing," "blaring, grinding noise," "as if he’d walked through some invisible wall cutting straight through his [hotel] room," preceding the onset of his symptoms. Many of the reports also indicated that the areas affected by these sounds were focused in a specific areas of their dwellings; moving out of the affected area caused the noise to stop. Reports such as this led the initial investigation into the phenomenon to suspect a sonic weapon was being used; however, the symptoms of mild brain trauma are prompting investigators to rethink this theory.

"Brain damage and concussions, it’s not possible," explains psychoacoustics expert Joseph Pompei, formerly with MIT. "Somebody would have to submerge their head into a pool lined with very powerful ultrasound transducers."

Aside from being baffled by the nature of the device used to carry out the attacks, the FBI is also still in the dark regarding who the culprit might be — the Cuban government, a rogue faction of hard-liners from its security forces, or possibly even Russia are all potential suspects.

"The investigation into all of this is still under way. It is an aggressive investigation," says State Department spokesperson Heather Nauert. "We will continue doing this until we find out who or what is responsible for this."