The US State Department is looking into what could be a new "sonic attack" in China, "similar to what happened in Cuba" over a year ago, a US diplomatic official told CNN.
A US government employee stationed in the city of Guangzhou in southern China reported "subtle and vague, but abnormal, sensations of sound and pressure," which may resemble sensations among at least two dozen US diplomats and family members previously stationed in Havana, Cuba.
After hearing bizarre sounds, Americans in China and Cuba experienced similar symptoms resembling types of brain injury
Experts don't think the sound was directly responsible for the symptoms
The cause in both cases is still being investigated
"We are working to figure out what took place both in Havana and now in China, as well," said Secretary of State Mike Pompeo, who called medical details in both locations "very similar and entirely consistent" with each other.
In both cases, people experienced symptoms resembling types of "traumatic brain injury," according to multiple sources.
What did they hear?
Many of those stationed in Havana reported hearing "intensely loud" sounds coming from a specific direction, which they described as "buzzing," "grinding metal," "piercing squeals" and "humming," according to a study published in February.
"The sounds were often associated with pressurelike or vibratory sensory stimuli," according to the study. "The sensory stimuli were likened to air 'baffling' inside a moving car with the windows partially rolled down."
One patient reported hearing two 10-second pulses, while others said they could hear the sound for more than 30 minutes, the report said.
A recording obtained by The Associated Press and released in October became the first publicly reported audio sample said to be related to the attacks.
Can sound really cause brain injury?
The noise itself is unlikely to have caused the symptoms directly, according to the authors of the February study. They noted that audible sound "is not known to cause persistent injury to the central nervous system."
"We actually don't think it was the audible sound that was the problem," said Dr. Douglas Smith, one of the study's authors and the director of the University of Pennsylvania's Center for Brain Injury and Repair. "We think the audible sound was a consequence of the exposure."
Experts whom CNN spoke to described how sounds could be weaponized, but none of them is known to cause the symptoms seen in Cuba and China.
For example, authorities have used long-range acoustic devices, or LRADs, to disperse crowds of protesters with a loud, painful sound over a long distance. Some countries have used a "mosquito" -- which produces a very high-pitched sound that can be perceived by teenagers but not adults -- to prevent teens from loitering.
"I know of no acoustic effect that would produce concussion-like symptoms; according to my research, strong effects on humans require loudness levels that would be perceived as very loud noise while exposed," said J-rgen Altmann, a physics professor at Technischen Universit-t Dortmund in Berlin.
According to the alert issued by the State Department on Wednesday, the cause of the injuries to the employee in China remains unknown. Officials have not identified any other similar cases among the diplomatic community in the country.
Similarly, State Department and federal investigators have testified that they were unable to determine the source or cause of the ailments in Havana, stating only that they "were most likely related to trauma from a non-natural source."
Smith and his colleagues wrote that their study raises "concern about a new mechanism for possible acquired brain injury from an exposure of unknown origin."
What were the symptoms?
"If you took any one of these patients and put them into a brain injury clinic and you didn't know their background, you would think that they had a traumatic brain injury from being in a car accident or a blast in the military," Dr. Randel Swanson, another of the study's authors and a specialist in brain injury rehabilitation at the University of Pennsylvania, said in a separate report published with the study.
Swanson and his colleagues examined the patients and found a variety of symptoms including sharp ear pain, headaches, ringing in one ear, vertigo, disorientation, attention issues and signs consistent with mild traumatic brain injury or concussion.
In addition, a majority of the patients reported problems with memory, concentration, balance, eyesight, hearing, sleeping or headaches that lasted more than three months.
"It's like a concussion without a concussion," Swanson wrote.
Many reported feeling "mentally foggy" or "slowed" for months, the authors said. Some reported irritability and nervousness, with two meeting criteria for post-traumatic stress disorder. Poorer job performance was also observed.
Three people eventually needed hearing aids for moderate to severe hearing loss, and others had ringing or pressure in their ears. More than half needed to be prescribed medication in order to sleep or to deal with headaches. Many were, at least for a period of time, unable to return to work.
Some of the patients' symptoms are not typically seen in a concussion, such as pain and ringing in only one ear, the study noted.
The report also pointed out that while concussion patients often make a quick and full recovery, these patients experienced symptoms for months.
It's possible those were the only severe cases, the authors said, noting that other individuals may have been affected and simply didn't know it, because they either recovered fully or had only minor symptoms.
Doctors remain baffled.
The events in Cuba and China are still under investigation, according to Pompeo.
A spokeswoman at the US Embassy in Beijing said the State Department was taking the incident "very seriously" and was working to determine the cause and impact of it. Pompeo said the agency is moving medical teams into place in Guangzhou and has asked for assistance from the Chinese government, which has committed to providing it.
The Cuban government has claimed ignorance when it comes to the cause of the incidents there, and it has aimed to discredit the accounts of affected government workers, calling the alleged attacks "science fiction."
These efforts have been met with doubt from Sens. Marco Rubio and Bob Menendez, both on the Foreign Relations Committee.
The idea that such an attack would go unnoticed by Cuban security officials in heavily surveilled Havana, Rubio said, was "outside the realm of reasonable. It's ridiculous."