The findings by the expert panel, which was convened by U.S. intelligence agencies, are the latest attempt to solve the years-long mystery of what, or who, is behind a constellation of symptoms known as Havana syndrome. In late 2016, personnel at the U.S. Embassy in Cuba’s capital reported a range of sensations including ringing and pressure in the ears, headaches and dizziness. Personnel in China later experienced similar symptoms, which have now been reported by hundreds of people serving at official posts around the world.

The vast majority of those cases have been attributed to medical conditions or other environmental factors, officials have said.

An earlier interim report from the CIA found that a foreign country is probably not mounting a global attack aimed at U.S. personnel. People who say they are victims of Havana syndrome strongly criticized the agency’s findings.

But the CIA report didn’t expressly rule out a foreign hand, at least behind some small number of cases for which investigators have found no cause or plausible explanation.

The panel of experts finding was consistent with earlier conclusions from the National Academies of Sciences, Engineering and Medicine, which found that “directed, pulsed radio frequency energy appears to be the most plausible mechanism in explaining these cases.”

That study was chaired by David Relman, a professor of microbiology and immunology at Stanford University, who also worked with the intelligence community’s panel of experts. The panel did not attempt to attribute any cases to a specific device or country.

The experts included people from within and outside the U.S. government, with expertise in science, medicine, and engineering. They were given access to classified government information on reported incidents and trends, and met with individuals who shared their personal experiences and medical records, according to intelligence officials familiar with the panel’s work, who briefed reporters on condition of anonymity under ground rules established by the intelligence agencies.

As a starting point, the panel examined “the plausibility of five potential causal mechanisms” for the syndrome, which government officials have termed “anomalous health incidents.” These included acoustic signals, chemical and biological agents, ionizing radiation, natural and environmental factors, and radio frequency and other electromagnetic energy.

In the end, the experts determined that “pulsed electromagnetic energy, particularly in the radio-frequency range, plausibly explains the core characteristics” of the health incidents. That finding was not definitive, and “information gaps exist,” the panel wrote in a summary of its findings. But “there are several plausible pathways involving various forms of pulsed electromagnetic energy, each with its own requirements, limitations, and unknowns” that could be making people sick.

Sources of energy exist, the experts wrote, that “could generate the required stimulus” on the human body, and that could be concealed and have “moderate power requirements,” suggesting that the energy could come from a portable device.

Such a device would apparently not be common, but it could be effective. “Using nonstandard antennas and techniques, the signals could be propagated with low loss through air for tens to hundreds of meters, and with some loss, through most building materials,” the summary stated.

Some people who have reported the symptoms said they came on suddenly while they were in offices and hotel rooms. And others have reported the onset while they were outside.

“Ultrasound also plausibly explains the core characteristics,” the experts found, but probably only at close range. The experts’ analysis of ultrasound suggested it might not explain the cases where people fell ill inside a building.

“Ultrasound propagates poorly through air and building materials, restricting its applicability to scenarios in which the source is near the target,” the experts wrote.

Skeptics of a directed-energy attack, perhaps with some form of weapon or surveillance device, have raised the possibility that Havana syndrome victims are actually suffering from some mass delusion or a psychological condition.

But the experts panel cast doubt on that hypothesis.

“Psychosocial factors alone cannot account for the core characteristics, although they may cause some other incidents or contribute to long-term symptoms,” they wrote.

Those core characteristics were helpful in narrowing down the investigation and focusing on aspects of symptoms that “were particularly difficult to explain through other means,” the experts wrote. There were four characteristics, including sound or pressure in the ears; “nearly simultaneous” symptoms such as vertigo, loss of balance and ear pain; “a strong sense of locality or directionality” to those symptoms; and the absence of any known environmental or medical conditions that could have caused them.

The panel acknowledged that some of these symptoms are common in known medical conditions. But the combination of all four “is distinctly unusual and unreported elsewhere in the medical literature, and so far have not been associated with a specific neurological abnormality.”

What’s more, the “location dependence,” or the way symptoms can appear suddenly, and go away quickly, argues for a “stimulus that is spatially and temporally discrete.” All those signs appear to point to some source of energy that is aimed at someone and causes the effects people experienced, the summary shows

An advocacy group of people who say they were the victims of an attack said 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 the incidents.

In a joint statement, two top U.S. intelligence officials said that the investigation into the cause of the health incidents will continue.

“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 as our efforts continue. Nothing is more important than the well-being and safety of our colleagues,” Avril Haines, the director of national intelligence, and William J. Burns, the director of the CIA, said in a statement.

While investigators have not settled on a cause for the illness, what’s clear is that the overwhelming majority of people who reported symptoms were not the victims of an attack.

Since the first cases were reported at the embassy in Havana in 2016, government investigators have reviewed more than 1,000 cases of anomalous health incidents, a senior CIA official told reporters in January.

The majority of cases could be attributed to a preexisting medical condition or environmental or other factors, the CIA official said. “A few dozen” of those incidents, which the official called “the toughest cases,” could not be explained and would receive further scrutiny.

People familiar with the CIA’s investigation, as well as the work of the expert panel, said that last year, after officials began to encourage anyone suffering from symptoms to report them, a flood of people came forward, most of whom turned out to have some explicable illness or cause behind their symptoms. That kept researchers busy trying to separate wheat from chaff, and ultimately they have zeroed in on that handful of cases that have so far stumped investigators.

The intelligence officials familiar with the panel’s work declined to say how many cases they examined. But the remaining few that have stumped investigators may hold the best chance for ultimately attributing the syndrome to a specific device or country.