A panel of government and outside experts assembled by U.S. intelligence agencies to solve the “Havana syndrome” mystery has determined that “directed, pulsed radio frequency energy appears to be the most plausible mechanism in explaining these cases,” according to an unclassified version of their findings released Wednesday. 

The CIA concluded last month that most of the disorienting and sometimes debilitating health issues reported by U.S. spies and diplomats over the past five years were not part of a sustained attack by a foreign power. But the agency did say about two dozen cases could not be explained by undiagnosed medical conditions, stress, or other natural causes, and those “genuine and compelling” cases formed the basis of the new findings.

The panel was given access to classified information and met with people afflicted with the four core symptoms: sound or pressure in the ears; simultaneous vertigo, loss of balance, and earaches; “a strong sense of locality or directionality” to the symptoms; and no known medical or environmental explanation. “Pulsed electromagnetic energy, particularly in the radio-frequency range, plausibly explains the core characteristics” of Havana syndrome, the panel wrote.

The National Academies of Sciences, Engineering, and Medicine reached a similar conclusion in 2020, finding that “directed, pulsed radio frequency energy appears to be the most plausible mechanism in explaining these cases.” 

The expert panel did not consider or speculate on who might be behind any directed-energy attacks, but it found that non-standard antennas fueled by moderate power sources could produced such symptoms from long distances and through the walls of buildings. “Ultrasound also plausibly explains the core characteristics,” the experts found, though it travels “poorly through air and building materials, restricting its applicability to scenarios in which the source is near the target.”

Along with radiofrequency and other electromagnetic energy sources, the expert panel considered acoustic signals, chemical and biological agents, natural and environmental factors, and radiation. 

NBC News correspondent Ken Dilanian reports that some CIA agents are refusing to serve overseas because they are frightened of being left debilitated by Havana syndrome.

“This is another major twist in the long-running Havana syndrome mystery,” BBC News reports. “Last month’s CIA study led many to conclude that perhaps there was less sign of any hostile activity and that the causes were medical or psychological conditions,” but “this panel restores the possibility of some kind of malicious activity back to the agenda.”