A U.S. government research study published on Monday in the medical journal JAMA found no significant physical evidence of brain injury among a group of federal employees suffering from the mysterious "Havana syndrome" ailment that first emerged in 2016, Reuters reports.

The study, conducted by researchers at the National Institutes of Health (NIH), compared 86 employees and adult family members reporting unusual health incidents with a control group of healthy volunteers with similar work assignments. Comprehensive clinical evaluations, including auditory, balance, visual, neuropsychological and advanced brain imaging tests, revealed few measurable differences between the two groups.

"These individuals have symptoms that are real, distressing and very difficult to treat," said Dr. Leighton Chan, the study's lead author and acting chief scientific officer at the NIH Clinical Center. "However, our findings did not identify a mechanism to explain their symptoms."

The "Havana syndrome" initially gained attention when U.S. embassy staff in Cuba reported experiencing strange sensory phenomena, such as hearing directional noise and feeling head pressure. This was often followed by headaches, dizziness, and cognitive difficulties. Similar cases later emerged among diplomats, intelligence officers and other personnel stationed worldwide, from China to Vienna.

While the study's results failed to find physical evidence of injury, lawyer Mark Zaid, who represents some of the afflicted individuals, cautioned that the findings "do nothing to undermine the theory that a foreign adversary is harming U.S. personnel and their families with a form of directed energy."