1,000 people linked to US embassies worldwide have reported symptoms of the so-called "Havana Syndrome."
The CIA reportedly said it doesn't think a foreign power caused it, but is investigating.
An intelligence report this week said the illness may be caused by "pulsed electromagnetic energy."
The mysterious illness known as "Havana Syndrome" may be caused by a sort of "pulsed electromagnetic energy," a panel of US intelligence experts said.
More than 1,000 officials, employees, and families linked to US embassies and missions around the world have reported symptoms of the illness, which include headaches, vertigo, and sound heard in only one ear.
The first case was reported in Havana, Cuba, in 2016 but cases have since been reported in countries including France, Austria, Switzerland, and Vietnam. Two suspected cases were also found in Washington, DC.
On Tuesday, the Intelligence Community Experts Panel on Anomalous Health Incidents released a report on the illness and its likely causes, in which it said Havana Syndrome may be caused by "pulsed electromagnetic energy, particularly in the radio frequency range."
"Ultrasound also plausibly explains the core characteristics, but only in close-access scenarios and with information gaps," it said.
The panel added that it was possible the illness it could be caused by an external stimulus with "moderate power requirements," suggesting an electronic device.
The panel did not assess who was responsible for the attacks or why people were targeted. However, CIA officials said last month that the agency believed it was unlikely that a foreign power was responsible for the attacks.
"We assess it is unlikely that a foreign actor, including Russia, is conducting a sustained, worldwide campaign harming US personnel with a weapon or mechanism," a senior CIA official told The Washington Post.
However, that official said the agency would continue to investigate who was responsible.
The Intelligence Community Experts Panel panel said it received dozens of briefings, more than 1,000 classified documents, and anomalous health incident reports.
The panel ruled out "ionizing radiation, chemical and biological agents, infrasound, audible sound, ultrasound propagated over large distances, and bulk heating from electromagnetic energy" as explanations.
The panel also said that the illness did not appear to be a result of hysteria or delusion, as some people have suggested.
The panel's assessment matched that given by the National Academies of Sciences, Engineering and Medicine in December 2020.
"Directed, pulsed radio frequency energy appears to be the most plausible mechanism in explaining these cases," that report said.
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