American intelligence agencies have not found a link between opposing countries and the so-called "Havana Syndrome" attacks. American personnel outside the United States have reported several hundred cases of health problems, mostly in the brain, including trauma, anxiety and depression.
The report released Wednesday by US intelligence officials cast doubt on earlier allegations that Russia or another country may be behind a global campaign to harass or attack Americans using some form of controlled energy.
Most of the cases investigated appear to have a variety of causes, from environmental factors to undiagnosed illnesses, said officials, who said they have not found a single explanation for most or all of the reported cases.
Intelligence officials say there is evidence that foreign countries were not involved.
In some cases, the United States revealed confusion among opposing governments regarding claims and suspicions that the "Havana Syndrome" was an American conspiracy. Investigators found "no credible evidence" that any adversary country had a weapon that could cause the reported symptoms or a listening device that could inadvertently injure people.
President Biden's administration has been under pressure to respond to so-called "Havana Syndrome" cases from US government employees who have reported injuries, their lawyers and US lawmakers.
President Joe Biden last year signed the HAVANA law, which provides compensation to individuals who are believed to have suffered injuries that the government describes as "abnormal health incidents."
Mark Zaid, an attorney for more than two dozen individuals who have reported injuries, said the new assessment lacked transparency by not answering key questions.
"Until the veils of secrecy are lifted and the analyzes that led to these conclusions are made available and subjected to proper scrutiny, the purported conclusions are invalid," he said in a statement. "But the damage it has caused to the morale of the victims, mainly by avoiding the government's failure to evaluate all the evidence, is real and must be punished."
Two officials with knowledge of the report briefed reporters on Wednesday on condition of anonymity, under rules set by the US Director of National Intelligence.
Investigators reviewed nearly 1,500 cases in 96 states. Many of the cases, officials said, are linked to other possible explanations such as illness, malfunctioning ventilation systems, or electromagnetic waves from computer equipment.