As the U.S. government considers closing its embassy in Cuba, scientists remain baffled by the mysterious sonic weapon that was apparently used to target American diplomats in Cuba.

On Sunday, Secretary of State Rex Tillerson said that the U.S. is considering the closure of its embassy in the Cuban capital following a spate of unexplained incidents that have left American diplomats injured.

Of the 21 medically confirmed U.S. victims, some have permanent hearing loss or concussions, while others suffered nausea, headaches and ear-ringing. Some are struggling with concentration or common word recall, the Associated Press has reported.

SONIC WEAPONS REPORTED IN CUBA: COULD SOUND BE HARNESSED IN AN ATTACK?

Some victims felt vibrations or heard loud sounds mysteriously audible in only parts of rooms, leading investigators to consider a potential "sonic attack." Others heard nothing but later developed symptoms.

The U.S. hasn't identified either a culprit or a device. Investigators have explored the possibility of sonic waves, an electromagnetic weapon, or an advanced spying operation gone awry, U.S. officials briefed on the probe told the AP. The U.S. hasn't ruled out that a third country or a rogue faction of Cuba's security services might be involved.

The strange incidents continue to puzzle scientists. There has been speculation that infrasound (a low frequency sound below the human hearing range) or ultrasound (above 20 KHz and can not be heard by humans), may have been harnessed in the attacks.

SOUND WEAPONS THAT DELIVER INVISIBLE AMMO IN SPOTLIGHT AFTER AMERICAN DIPLOMATS INJURED

“Ultimately, devices working in either spectrum could cause hearing damage but it is unlikely to be infrasonic given the size of the speaker required to produce the requisite frequency and decibel level,” explained Dr. Toby Heys, leader of Manchester Metropolitan University’s Future Technologies research centre, in a statement emailed to Fox News. “Infrasound is also very difficult/next to impossible to direct, within current technological dictates.”

"There's no efficient way to focus infrasound to make it into a usable weapon," said Mario Svirsky, an expert on ear disorders and neuroscience at New York University School of Medicine.

Heys added that ultrasound could be directed at a target’s head, but would require extremely precise targeting within a building’s infrastructure.

SONIC WEAPON KNOCKS DRONES RIGHT OUT OF THE SKY

Experts say that it is also difficult to explain the concussions experienced by some of the victims of the attacks. Usually, those follow a blow to the head or proximity to something like a bomb blast.

“The frequency-based trauma in question would have to be very severe to cause this,” explained Heys. “There is a history of brain injury and hearing loss that stretches back to WW1 when soldiers had to endure not only the intense and massive sounds of conflict for extended periods of time but also the shock waves produced by large artillery.”

The academic cited a French study in 1918 that concluded, that, when exposed to intense noise over a prolonged period of time, the skull does not particularly protect the brain. “The difference of course here is that this is audible sound, whereas the frequencies in question in Cuba are supposedly ‘non-audible’, which makes it more difficult to explain as a phenomenon,” he added.

The Associated Press contributed to this article.

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