In late 2016, dozens of United States diplomats working in Cuba and China began reporting odd mental symptoms: persistent headaches, vertigo, blurred vision, hearing phantom sounds. Since then, scientists and commentators have groped for plausible explanations. Deliberate physical attacks, involving microwaves or other such technology? Or were psychological factors, subconscious yet mind-altering, the more likely cause?

The strangeness of the symptoms, and the spookiness of the proposed causes, have given the story a life of its own in the diplomatic corps, the Pentagon and in assorted pockets of the internet where conspiracy theories thrive.

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Now, researchers are reporting results from the first brain-imaging studies of 40 of those diplomats, who were carefully examined by neurologists after returning home from Cuba. The study, appearing on Tuesday in the medical journal JAMA, concludes that the diplomats experienced some kind of brain trauma. But the nature and cause of that trauma were not clear, as it did not resemble the signature of more familiar brain injuries such as repeated concussions or exposure to battlefield blasts.

“The main thing we can do with brain imaging is ask whether something happened to the brain,” said Ragini Verma, a professor of radiology at the University of Pennsylvania Perlman School of Medicine and the lead author of the new report. “And the answer we found is that yes, it did.”