“Pulsed electromagnetic energy, particularly in the radio-frequency range, plausibly explains the core characteristics” of the reported symptoms,” it says, while adding that “information gaps exist.”

The report by a group convened last year by Director of National Intelligence Avril Haines and CIA Director

William Burns

adds to a growing body of evidence suggesting that while many of the roughly 1,000 reported cases of Havana Syndrome can be explained by stress, existing medical conditions or other issues, others cannot.

The new report differs—at least in tone and emphasis—from an interim CIA report released two weeks ago that deemed it unlikely that Havana Syndrome was the result of a sustained campaign of attacks on U.S. personnel by a foreign adversary such as Russia. That report angered victims of the syndrome, who welcomed the conclusions Wednesday by the expert panel.

The group convened by Ms. Haines and Mr. Burns didn’t single out a specific cause behind the reported attacks, and wasn’t mandated to examine who might be conducting them.

Havana Syndrome is a set of unexplained medical symptoms including headaches, dizziness, fatigue, nausea, anxiety and cognitive difficulties. Known in the U.S. government as “anomalous health incidents,” they were first experienced by State Department personnel stationed in Cuba in late 2016. Since then, cases have been reported among U.S. diplomats and intelligence officers globally, including in major European cities, Colombia and Vietnam.

When the conclusions of the interim CIA report were made public last month, a senior CIA official said that while the majority of reported cases could be explained by pre-existing medical conditions or other factors, a few dozen couldn’t be and would be investigated further.

While the CIA task force was charged with examining who might be attacking U.S. personnel and why, the intelligence community experts panel focused on possible mechanisms or devices that would explain the symptoms victims have reported. Its finding that electromagnetic or acoustic devices could be responsible for some of the cases, however, suggest that someone is deliberately harming U.S. personnel overseas.

Members of a group representing individuals who say they have been afflicted with Havana Syndrome and who had harshly criticized the CIA conclusions welcomed the latest report’s release.

The new report “reinforces the need for the intelligence community and the broader U.S. government to redouble their efforts to fully understand the causes of Anomalous Health Incidents, or ‘Havana Syndrome,’” the group Advocacy for Victims of Havana Syndrome said. “We, our families and colleagues, and the nation would have been far better served by a coordinated presentation of the findings of this panel and CIA’s.”

The panel, which included scientific, medical and engineering experts from outside government as well as intelligence analysts, reviewed more than 1,000 classified documents, received dozens of briefings, analyzed medical reports, and interviewed as many as 20 victims, an intelligence official familiar with the group’s work said.

It looked at five possible causes for the symptoms: acoustic devices, chemical or biological agents, ionizing radiation, natural and environmental factors, and electromagnetic energy, according to the report’s partially declassified executive summary.

“Some incidents have affected multiple persons in the same space, and clinical samples from a few affected individuals have shown early, transient elevations in biomarkers suggestive of cellular injury to the nervous system,” the report found.

A subset of Havana Syndrome cases, it says, involve four characteristics that, taken together, don’t appear to be caused by any known medical condition. Those are the acute onset of audio-vestibular phenomena such as pressure in one ear or on one side of the head, other near-simultaneous symptoms such as vertigo, a sense that the phenomenon is localized or coming from a specific direction, and the absence of known environmental or medical factors that would explain these symptoms.

Intelligence officials familiar with the panel’s work declined to say how many cases out of the roughly 1,000 reported fall into this subset.

Pulsed electromagnetic energy, particularly in the radio-frequency range, plausibly explains the symptoms in the subset of cases, the summary says. “Sources exist that could generate the required stimulus, are concealable, and have moderate power requirements,” it says.

That finding aligns with the conclusions of a December 2020 National Academy of Sciences, Engineering, and Medicine study that concluded that exposure to some type of directed energy was most likely the culprit for a number of symptoms associated with Havana Syndrome.

Some type of ultrasound device also could explain the symptoms, the intelligence community experts panel found, although such a device would have to be in proximity to the victim.

Scientific researchers exposed to radio-frequency signals or to high-power ultrasound beams have experienced some of the same sensations reported by Havana Syndrome victims, the executive summary says.

Ms. Haines and Mr. Burns said in a joint statement that probes into Havana Syndrome will continue.

“We will stay at it, with continued rigor, for however long it takes,” they said. “The U.S. government remains committed to providing access to care for those who need it, and we will continue to share as much information as possible with our workforce and the American public.”

Write to Warren P. Strobel at Warren.Strobel@wsj.com