Repeated scans of patients suffering from the mysterious ailment commonly known as “Havana syndrome” found no significant evidence of brain injury, according to an ongoing investigation by the National Institutes of Health.

Two studies, published Monday in JAMA, found few significant differences in a range of cognitive and physical tests among more than 80 patients who had been stationed in Cuba, Austria, China and other locations compared with a control group of people with similar job descriptions.

The researchers said they did not seek to find the origin of the ailments, which the U.S. government refers to now as “anomalous health incidents” (AHIs). Nor did the NIH researchers seek to disprove conjectures, which have received extensive media coverage, that American officials were attacked by an unidentified foreign adversary using some kind of newly invented pulsed-energy weapon.

The focus instead was on the physical condition of the patients, including whether they showed signs of injuries or brain damage.

The new findings are poised to reignite the controversy over this now-global medical mystery, which sparked a rash of inconclusive investigations, roiled the State Department and Central Intelligence Agency and heightened tensions between the United States and Cuba. While the NIH study is unlikely to resolve the politically charged debate, it could intensify doubts in the scientific and intelligence communities about the existence of unidentified adversaries targeting government officials with an exotic weapon.

The researchers emphasized that the patients who volunteered for the study, which began in 2018, do have severe symptoms that can be debilitating.

“These symptoms are real, and they feel it, and we acknowledge it,” said Leighton Chan, lead author of one of the papers. “We have a third of our cases that either are not working or are struggling to work.”

The researchers believe their findings should be viewed as good news, because these patients do not show evidence of brain damage, and many are already showing signs of improvement in their symptoms.

Two earlier studies, also published in JAMA, reported that brain scans of some patients showed unusual characteristics compared with controls. The NIH reports said the new data do not support those earlier findings.

Real symptoms, unknown cause

The JAMA papers are the latest attempt by medical and national security experts to explain the rash of ailments that got its nickname after a number of State Department officials in late 2016 reported mysterious symptoms while stationed in or near the U.S. Embassy in Havana.

The patients complained of intrusive sounds and head pressure, followed by a broad range of symptoms, including dizziness, pain, blurry vision, tinnitus, fatigue, nausea and cognitive dysfunction. Some said they heard a buzzing or high-pitched sound immediately before feeling ill.

As media reports proliferated, some national security officials speculated that Russia or some other adversary had developed a secret pulsed-energy weapon. Then Havana syndrome went global, with reports in Europe and Asia of American officials suffering a variety of symptoms due to “targeted attacks,” as then-Secretary of State Rex Tillerson put it.

There were instant and protracted political consequences of the medical mystery. The Trump administration in 2017 responded by expelling Cuban diplomats and pulling back on what had been a policy of engagement with the communist-run island under the Obama administration.

The State Department and intelligence agencies launched what became a multiyear effort to find the alleged perpetrators. But the search for malefactors came up empty. In March 2023, the intelligence community concluded that no foreign adversary was behind the phenomenon and that AHIs were not caused by an energy weapon or as the byproduct of some other activity, including electronic surveillance that could have made people sick unintentionally.

Intelligence officials have said they remain open to new ideas and evidence. For instance, if information emerged that a foreign adversary had made progress developing the technology for an energy weapon, that might cause analysts to adjust their assessments.

Further investigations cast more doubt on whether there were attacks of any kind. But the politics of Havana syndrome have made skepticism tricky. And the earlier JAMA studies, as well as two other government-sponsored research inquiries, fueled speculation that the patients had been attacked with a weapon emitting pulses of electromagnetic energy.

Between 2018 and 2021, the NIH study team recruited federal workers and their adult family members who reported AHIs. They continued collecting data until 2022. Of the volunteers, 86 were tested for psychological conditions along with vision, hearing, balance and blood markers, and 81 of them also were studied with MRI scans to look for brain injuries. The results were then compared with the control group.

“When we tested cognitive function, we didn’t see those deficits,” said neuropsychologist Louis French, deputy director of the National Intrepid Center of Excellence at Walter Reed National Military Medical Center and a co-investigator on the study. “What you’re seeing in some cases here are people that, for various reasons, have ongoing symptoms they’re reporting that just don’t play out when we’re looking at this objectively.”

The absence of signs of brain injuries does not rule out the possibility of an external adverse event causing the symptoms, said Carlo Pierpaoli, the lead author on the neuroimaging paper.

In a sign that the research has yet to resolve the issue, JAMA also published an editorial by Stanford University professor of medicine David Relman, who led two previous studies that said the AHIs “could be plausibly explained by exposure to directed, pulsed radio-frequency energy.”

Although the “NIH study used advanced neuroimaging techniques, current MRI technology may be insensitive to, or improperly timed for, the kinds of potentially transient cellular and local physiological disruption that characterizes AHIs,” Relman wrote in the editorial.

And Mark Zaid, an attorney who represents current and former federal officials and their family members who say they have been afflicted with AHIs, cast doubt on the new studies.

“Frankly, they reveal very little beyond the assertion they could not identify any differences between the AHI and non-AHI population,” Zaid said in a statement. “Without a doubt the Intelligence Community is going to point to these studies to say ‘see, we told you, nothing there.’ But the absence of evidence is not evidence.”