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No one knows exactly what caused 24 U.S. diplomats and their families in Cuba to fall ill and in many cases show signs of brain injuries after they reported hearing strange noises.

But whatever is causing these mysterious illnesses, it seems to now be happening in China too.

On May 23, the U.S. State Department announced that one embassy worker in Guangzhou experienced “subtle and vague, but abnormal, sensations of sound and pressure” before being diagnosed with symptoms similar to those found in the diplomatic personnel that were in Cuba, including mild traumatic brain injury.

The New York Times reported Wednesday that at least two more Americans in Guangzhou have experienced similar phenomena and also fallen ill. One of those embassy workers told the Times that he and his wife had heard mysterious sounds and experienced strange headaches and sleeplessness while in their apartment.

Related: What We Know About The Mysterious Sonic Weapons Sickening US Diplomats In Cuba »

After the evacuation of the first diplomatic employee from Guangzhou was announced, the State Department issued a health alert via the U.S. Consulate in Guangzhou telling people that “if you experience any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises, do not attempt to locate their source. Instead, move to a location where the sounds are not present.”

On June 5, the office of U.S. Secretary of State Mike Pompeo announced the establishment of a task force meant to respond to these mysterious incidents, which some have called “sonic attacks.”

“U.S. government personnel and family members at affected locations have been directed to alert their mission’s medical unit if they note new onset of symptoms that may have begun in association with experiencing unidentified auditory sensations,” the State Department announcement said. “Reported symptoms have included dizziness, headaches, tinnitus, fatigue, cognitive issues, visual problems, ear complaints and hearing loss, and difficulty sleeping.”

A mysterious problem that began in Cuba

The saga began in late 2016, when American diplomatic staff (and some Canadians) that had been in Cuba began to report odd physical and mental symptoms. Some individuals could no longer remember words, while others had hearing loss, speech problems, balance issues, nervous-system damage, headaches, ringing in the ears, and nausea.

Some even showed signs of brain swelling or concussions — mild traumatic brain injuries.

A study of those victims suggested a disconcerting possibility: some unknown force projected in the direction of the patients could have somehow injured their brains.

“The unique circumstances of these patients and the consistency of the clinical manifestations raised concern for a novel mechanism of a possible acquired brain injury,” the study’s authors wrote.

Many of the victims remembered strange occurrences before the symptoms appeared, though others didn’t hear or feel anything. One diplomat reported hearing a “blaring, grinding noise” that woke him from his bed in a Havana hotel, according to the Associated Press. The AP also reported that some heard a “loud ringing or a high-pitch chirping similar to crickets or cicadas” in short bursts at night, while others said they could walk “in” and “out” of blaring noises that were audible only in certain spots.

The US State Department eventually determined that the incidents were “specific attacks” and moved to cut its Cuban embassy staff by 60%.

The recent State Department announcement said there have been at least 24 victims of these attacks. Of those, 21 were studied by a team of researchers from the University of Pennsylvania’s Center for Brain Injury and Repair. More than 80% reported hearing a sound that had a “directional” source — it seemed to come from somewhere.

After three months, 81% still had cognitive issues, 71% had balance problems, 86% had vision issues, and about 70% still reported hearing problems and headaches.

The fact that a number of these symptoms could be subjective has raised questions about the possibility that this group of people is suffering from some sort of collective delusion, according to the study authors. But they say that mass delusion is unlikely, since affected individuals were all highly motivated and of a broad age distribution, factors that don’t normally correspond with mass psychogenic illness. Plus, objective tests of ears and eye motion all revealed real clinical abnormalities.

The symptoms seem consistent with some form of mild brain trauma, according to the researchers. But these symptoms persisted far longer than most concussion symptoms do, and were not associated with blunt head trauma.

“These individuals appeared to have sustained injury to widespread brain networks without an associated history of head trauma,” the study authors wrote.

An unknown cause

Despite having identified common symptoms and clinical evidence of some sort of injury, researchers are still at a loss about what happened to these diplomats.

If there is some kind of weapon involved, no one knows what kind it was or who would have used it. The Cuban government denied any connection and investigators hadn’t found a link to Russia, which intelligence analysts have speculated might have the means and motivation to carry out such an attack.

Now that there are cases in China, the mystery is even deeper.

The reported presence of strange audio and of the feeling of changes in air pressure have led to speculation about some kind of sonic or audio-based weapon. But although sonic weapons exist, they’re very visible and easy to avoid, according to Seth Horowitz, a neuroscientist who wrote the book “The Universal Sense: How Hearing Shapes the Mind. Plus, the specific symptoms make a sonic weapon unlikely.

“There isn’t an acoustic phenomenon in the world that would cause those type of symptoms,” Horowitz said.

Related: Listen To The First Recording Of The ‘Sonic Weapon’ Sickening US Diplomats In Cuba »

He speculated that perhaps some sort of mysterious pathogen or other phenomenon could have caused the symptoms, but the authors of the study on the victims from Cuba reported found no signs of infection (like fever). They determined that it was unlikely a chemical agent caused these effects, since it would have damaged other organs, too.

In an editorial published alongside that study, two doctors wrote that without more information and more data on the patients before they reported feeling ill, they couldn’t definitively figure out what went wrong.

“At this point, a unifying explanation for the symptoms experienced by the U.S. government officials described in this case series remains elusive and the effect of possible exposure to audible phenomena is unclear,” the editorial’s authors wrote. “Going forward, it would be helpful for government employees traveling to Cuba to undergo baseline testing prior to deployment to allow for a more informed interpretation of abnormalities that might later be detected after a potential exposure.”

Now, employees headed to China may have to consider similar testing.

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