WASHINGTON (AP) — An array of advanced tests found no brain injuries or degeneration among U.S. diplomats and other government employees who suffer mysterious health problems once dubbed “Havana syndrome, ” researchers reported Monday.

The National Institutes of Health’s nearly five-year study offers no explanation for symptoms including headaches, balance problems and difficulties with thinking and sleep that were first reported in Cuba in 2016 and later by hundreds of American personnel in multiple countries.

But it did contradict some earlier findings that raised the specter of brain injuries in people experiencing what the State Department now calls “anomalous health incidents.”

“These individuals have real symptoms and are going through a very tough time,” said Dr. Leighton Chan, NIH’s chief of rehabilitation medicine, who helped lead the research. “They can be quite profound, disabling and difficult to treat.”