Last September, two Cuban government handlers took me to a neuroscience-research institute in Havana to meet with Mitchell Joseph Valdés-Sosa, a jovial scientist who emigrated to Cuba from the United States when he was eleven. Valdés-Sosa was part of a team of prominent Cuban medical experts that was set up by the government in response to American allegations that U.S. Embassy personnel in Havana, starting in late 2016, were targeted in mysterious attacks that caused serious brain injuries. During our hour-long conversation, Valdés-Sosa said that the Americans had offered no credible scientific evidence that “attacks” had taken place. He ridiculed U.S. speculation that a directed-energy weapon of unknown origin had somehow caused the injuries. His colleagues at the institute joked that it sounded as though officials in Washington were watching too many reruns of “The X-Files,” an American science-fiction television series that was also popular in Cuba. To underline their point, they started humming the show’s spooky theme music and then burst out laughing.

Given Valdés-Sosa’s role as one of the Cuban government’s handpicked spokesmen regarding the mystery incidents, he hardly offered an impartial assessment of what happened. But Valdés-Sosa is not the only skeptic in the scientific community arguing that the U.S. government should have seriously considered the possibility that the victims suffered from a mass psychogenic or psychosomatic event, one fuelled by warnings from their leaders that they were under attack. In recent months, this hypothesis has gained traction in some circles in Washington, in part because, after two years of investigating, both F.B.I. agents, who travelled to Cuba in search of evidence, and U.S. spy agencies, which have pored over intercepted communications looking for signs of foreign involvement, have yet to figure out what caused the injuries. “We still don’t know what happened,” a senior U.S. official told me. “We still don’t know what it is.” The affliction didn’t have a name, so some of the victims started to refer to it simply as “the Thing.”

One of the early victims was a fifty-year-old career Foreign Service officer, who asked to be identified as Audrey Lee, a pseudonym, to protect her identity. (Lee’s injuries were described in a story in this magazine, last November.) “I was personally disgusted by the fact that so many people were making judgments and assumptions about what happened to us without ever having met with any of us, without ever having examined us, or seeing our medical files,” Lee told me last week, in a phone call. “There’s been so much reporting about the possibility of everything being psychosomatic or being stress. And none of that really made sense to any of us. But, at the same time, we didn’t have proof, like an amputated limb. There was nothing obvious when you looked at us.”

Lee and her colleagues felt somewhat vindicated last week, when brain specialists at the University of Pennsylvania, who examined and treated forty U.S. government employees who suffered strange injuries while working at the Embassy in Havana, published their first brain-imaging findings in The Journal of the American Medical Association, or JAMA. The results showed tangible neurological changes that “are not like anything we’ve ever seen before,” Douglas Smith, the director of the university’s Center for Brain Injury and Repair and the investigative team’s leader, told me.

Smith and his team were following up on their preliminary findings, published by JAMA in February, 2018, which described what appeared to be a new type of “brain network disorder.” According to the first study, the victims appeared to have sustained serious brain injuries that resembled concussions suffered by soldiers struck by roadside bombs in Iraq and Afghanistan. But there were no signs of impact. One of the specialists at the University of Pennsylvania said that it was as if the victims had a “concussion without concussion.” After those preliminary findings were published, JAMA received letters from other medical specialists arguing that the study was flawed, especially in neglecting psychological explanations for the victims’ symptoms.

Normal imaging technology isn’t sensitive enough to show the impact of a concussion on brain matter, because the damage occurs to fibers, called axons, that are so thin that it would take a hundred of them, set side by side, to equal the width of a single strand of human hair. Axons create networks that transfer electrical signals around the brain. A white-colored fatty substance wraps around each axon, creating insulation, known as “white matter,” that speeds connectivity. Advanced studies of concussions and other traumatic-brain-injury cases often show changes in the white matter’s structure, connectivity, and volume. Smith’s team assumed that the patients who worked in Havana would have similar white-matter changes. To make that assessment, advanced neuroimaging machines were used to examine the brains of the forty victims, including Lee.

Based on the scans, Smith’s team concluded that the volume of white matter in the victims’ brains was “significantly smaller” than in a similar group of healthy adults. “It is clear that something structural in the brain was affected, but we don’t know what that is and what caused the effects,” Smith told me. (In a note that accompanied the University of Pennsylvania paper, editors at JAMA cautioned readers that the relevance of the neuroimaging findings was “uncertain” and that the exact nature of the affliction “remains unclear,” adding that more scientific evidence is needed.)

Despite the editors’ caution, Smith, a traumatic-brain-injury specialist, told me, “The mass-hysteria hypothesis is debunked by very obvious neurological findings that you can’t imitate, you can’t mimic.” Lee welcomed the results: “It was certainly validating to see that there are signs of brain damage.”

Most of the forty patients studied by the University of Pennsylvania team, a group that includes C.I.A. officers and State Department diplomats, have returned to work, Smith told me. The C.I.A. declined to comment, but the JAMA article said that the University of Pennsylvania team had received permission to include imaging data in its analysis from all of the victims except four, who “were unreachable by any mode of communication,” presumably because they were intelligence officers who, since leaving Havana, have started operating under new covers in other countries.

Lee’s health problems started on the night of March 17, 2017. She was standing in the kitchen of her home, in an upscale suburb of Havana, when, all of a sudden, she felt a burst of pressure in her head, followed by a stabbing pain worse than anything that she had ever experienced before.

After receiving treatment at the University of Pennsylvania, Lee was sent by the State Department, in 2018, to the U.S. Embassy in Barbados, for a new, three-year assignment managing consular affairs.

Despite the treatment she received, Lee’s condition wasn’t improving. “In fact, I was getting worse,” she told me. Since the mysterious event in her kitchen, Lee had suffered from severe headaches. Her balance problems persisted. Her vision problems were getting worse.

This year, during her assignment in Barbados, she started experiencing a new symptom, which she described as mental blackouts. “I don’t remember them. They’re described to me as this kind of shutting down. You can wave a hand in front of my face and I am not cognizant of it,” she said.