A preliminary CIA report concluded that a global campaign by a foreign power was likely not the cause of a mysterious illness known as Havana syndrome that's afflicted American and Canadian diplomats around the world, an agency official said.

Why it matters: A CIA official told Axios that most of the 1,000 cases reported to the government could be "explained by medical conditions or environmental and technical factors, including things like previously undiagnosed illnesses and the fact that many of the reports were made out of an abundance of caution."

• "We assess that it is unlikely that a foreign actor, including Russia, is conducting a sustained, worldwide campaign harming U.S. personnel with a weapon or mechanism," the official said.
• Some lawmakers had suggested the sometimes debilitating illness was due to directed energy attacks.

Yes, but: A CIA official said that "there's a subset of cases, our toughest cases so far, that remain unresolved and remain the focus of an active investigation." In those cases, the agency has "not ruled out the involvement of a foreign actor and continues to investigate," the official added.

• Illnesses from when Havana syndrome was first detected at the U.S. Embassy in Cuba's capital in late 2016, were among the roughly two dozen unexplained cases, officials told several outlets.

The big picture: A 2020 National Academies of Sciences, Engineering, and Medicine report found radio frequency energy of radiation that includes microwaves was likely behind the illness that's continued to hit diplomats, most recently in Geneva and Paris this month. Symptoms can include extreme headaches and nausea.

• The 2020 report does not attribute blame for the suspected attacks. But it notes there "was significant research in Russia/USSR into the effects of pulsed, rather than continuous wave [radio frequency] exposures" and that military personnel in "Eurasian communist countries" were exposed to non-thermal radiation.
• Russia has always denied being behind Havana syndrome.

What they're saying: In a written statement, CIA Director William Burns said that the agency was "pursuing this complex issue with analytic rigor, sound tradecraft, and compassion and have dedicated intensive resources to this challenge."

• "While we have reached some significant interim findings, we are not done," Burns added. "We will continue the mission to investigate these incidents and provide access to world-class care for those who need it."

Go deeper: Biden signs bill to support victims of mysterious "Havana Syndrome"

Editor’s note: This article has been updated with new details throughout.