The State Department says a US government employee working in China suffered "subtle and vague, but abnormal, sensations of sound and pressure" that later led to a diagnosis of "mild traumatic brain injury" — resulting in a warning to all US citizens in the country.
The strange incident recalls a similar spate of reports from Cuba, where US officials reported symptoms consistent with a "sonic attack," or exposure to harmful frequencies.
"The US government is taking these reports seriously and has informed its official staff in China of this event," the State Department warned in a health alert. "We do not currently know what caused the reported symptoms, and we are not aware of any similar situations in China, either inside or outside of the diplomatic community."
The State Department went on to advise: "While in China, 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."
Emily Rauhala, The Washington Post's China correspondent, reported that the State Department confirmed the US worker's ailment was diagnosed as a mild traumatic brain injury, something US officials in Cuba also experienced.
The U.S. Embassy in Havana, Cuba.
The Post reports that Chinese and US officials are looking into the matter. Americans working in Cuba suffered permanent hearing loss, severe headaches, loss of balance, brain swelling, and disruption to cognitive functions.
The US originally called the Cuba incidents "sonic attacks" but later backed off that phrasing as medical experts examined the patients and found their symptoms and conditions to be of mysterious origins.
Medical testing revealed the embassy workers in Cuba developed changes to the white-matter tracts that let different parts of the brain communicate, officials told the Associated Press.
But a purposeful attack hasn't been ruled out as the source of the brain injuries now linked to two countries.
"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 from a directional exposure of undetermined etiology," a study about the victims in Cuba concluded.
This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.
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