An intensive review by US intelligence agencies found no evidence linking any foreign adversaries to the so-called Havana syndrome, according to a recently declassified assessment.

«This is not simply an absence of evidence,» a senior US intelligence official told reporters at a briefing on the findings. «We have a lot of evidence that points in another direction.»

Intelligence officials said the findings released Wednesday were the culmination of a years-long effort to verify a long-standing hypothesis that an American adversary, most likely Russia, was responsible for at least some of the mysterious and sudden brain injuries suffered by a small group of American diplomats and spies abroad.

Last year, a provisional CIA assessment ruled out a global campaign by a foreign government, but said there were two dozen cases in which an attack could not be ruled out. Wednesday’s findings concluded that, even in those cases, there was no evidence pointing to a weapon or any other device used by a foreign adversary or anyone else.

Beginning in late 2016, U.S. diplomats and spies serving in Cuba began reporting strange sounds and sensations followed by unexplained illnesses and symptoms, including hearing and vision loss, memory and balance problems, headaches and nausea.

Over the years, about 15,000 US government employees have reported suspicious incidents in more than a dozen countries.

When asked what might have caused the documented brain injuries sustained by some of the employees, intelligence officials cited pre-existing medical conditions, environmental factors and an environment in which spies and diplomats perceived they were under mysterious attack.

The officials were careful to say they were not disputing or dismissing any of the symptoms cited by those who reported them. It remains to be seen what impact the assessment will have on a whole-of-government effort, mandated by Congress, to provide compensation to people who suffered what were officially deemed «abnormal health incidents.»

NBC News reported in 2018 that US intelligence officials considered Russia a prime suspect in what some judged to be deliberate attacks on CIA officers and diplomats abroad. But in the years since, the spy agencies have failed to develop any evidence pointing to Russia or any other adversary, or any deliberate act by anyone.

Intelligence collected on various foreign adversaries suggested they were confused by the claims and believed it was a US disinformation plot, an intelligence official said.

And a subsequent medical analysis determined that there was no coherent set of symptoms.

The Biden administration has been criticized by lawmakers for its response to health incidents. Diplomats who have reported symptoms have complained that the State Department bureaucracy has been slow to provide care, evaluation and benefits, and that some senior officials have sometimes shown skepticism about the syndrome.

In November 2021, Secretary of State Antony Blinken appointed a new diplomat to lead a task force overseeing the response to the “Havana syndrome” after the previous chief came under heavy criticism and resigned.