WASHINGTON — U.S. intelligence agencies have determined that a foreign adversary is “very unlikely” to be responsible for the mysterious ailments known as Havana syndrome that American spies and diplomats have reported experiencing at missions around the world since 2016, officials announced on Wednesday.
The assessment builds on interim findings from the C.I.A. last year that neither Russia nor another hostile power was responsible for a global campaign targeting intelligence officers and diplomats who reported a wide range of symptoms such as headaches, dizziness and balance problems. In many of these cases, the patients said the symptoms began after they heard a strange sound and felt intense pressure in their heads.
But the conclusions released by the Director of National Intelligence’s office on Wednesday were broader, finding that none of the episodes the government investigated could be attributed to hostile foreign action.
The intelligence community assessment found that while seven different agencies had varying levels of confidence, most “concluded it is ‘very unlikely’ a foreign adversary is responsible” for the reported ailments. As part of the investigation, U.S. spy agencies reviewed intelligence, which showed that adversaries were puzzled and thought the reported symptoms were part of an American plot.
Furthermore, the spy agencies concluded that there was no “credible evidence” any adversaries had developed a weapon or an intelligence-collection device cable to cause the injuries that American officials have reported. However, a team of experts at the Pentagon is continuing to investigate the matter.
The mysterious ailments have been referred to as Havana syndrome because the first known cases were reported by C.I.A. officers in the Cuban capital in 2016. Intelligence officers, diplomats and other U.S. government employees in China, Austria and dozens of other countries subsequently reported similar cases.
Victims groups immediately pushed back on the findings. Mark S. Zaid, a lawyer who represents several Havana syndrome patients, said that the assessment undermined morale and that the intelligence agencies needed to provide more details about their work.
“The latest U.S. intelligence assessment lacks transparency, and we continue to question the accuracy of the alleged findings,” he said.
William J. Burns, the C.I.A. director, said in a statement that the findings reflected more than two years of “rigorous, painstaking collection, investigative work and analysis” by the C.I.A. and other U.S. intelligence agencies.
“I and my leadership team stand firmly behind the work conducted and the findings,” Mr. Burns said. “I want to be absolutely clear: These findings do not call into question the experiences and real health issues that U.S. government personnel and their family members — including C.I.A.’s own officers — have reported while serving our country.”