Just a year after the United States reopened its embassy in Havana, Cuba, in 2015, some diplomats began experiencing a painful ringing in their ears. This “Havana syndrome” was first reported at that embassy in 2016, and included symptoms like dizziness and headaches, incapacitating diplomats and the spies who worked alongside them. It was reported across several embassies over multiple years.

One immediate hypothesis was that the symptoms were the result of a deliberate attack by another nation against the diplomats and spies of the United States. Today, that theory is as close to being fully dismissed as it has ever been. On March 1, the Washington Post reported that five US intelligence agencies determined that Havana syndrome was very unlikely to be the result of action by a foreign adversary. With the new reporting, while the actual origins of all injuries attributed to it cannot be pinned down, it is safe to say the majority of the intelligence community does not see the symptoms as resulting from malicious action by a hostile nation.

The intelligence community, as the collection of spy and surveillance agencies are known, includes better known agencies like the CIA, NSA, and the FBI, as well as long-running but lower-profile organizations like the Defense Intelligence Agency (DIA) or the National Geospatial-Intelligence Agency. Seven of these agencies (the Post’s reporting does not specify which ones) conducted a review of around 1,000 cases broadly identified under the Havana syndrome banner.

The Post, citing two intelligence officials who remained anonymous, summarized the findings this way: “Five of those agencies determined it was ‘very unlikely’ that a foreign adversary was responsible for the symptoms, either as the result of purposeful actions — such as a directed energy weapon — or as the byproduct of some other activity, including electronic surveillance that unintentionally could have made people sick,” wrote Shane Harris and John Hudson. 

Of the remaining two agencies on the review, one determined that it was merely “unlikely,” not “very unlikely” that a foreign government was responsible, while that last agency did not offer a conclusion either way. Still, none of the agencies in the review offered a dissenting view from the conclusion. 

Congress has already mandated payments for those injured by the syndrome, which the Biden administration last summer said it would honor, even as no clear cause of the symptoms could be found. 

The previous leading explanation was an as-yet undiscovered advanced directed energy weapon.

How would such a weapon work?

Directed energy weapons have moved from the realm of science fiction to reality. These include, most commonly, high-powered lasers and microwaves, which operate in different ways. Laser weapons need a clear line of sight, and cause harm by heating up the surface of the drone or other object they are in contact with, until that object burns or breaks. Popular Science even had the chance to try one.

Because Havana syndrome sufferers lacked visible marks, it is easy to rule out a laser as the origin. Other directed energy weapons, like high-power microwaves, or sound beyond what humans can consciously perceive, have also been considered and then dismissed as possible causes. 

[Related: What it’s like to fire Raytheon’s powerful anti-drone laser]

“The officials said that as analysts examined clusters of reported cases, including at U.S. embassies, they found no pattern or common set of conditions that could link individual cases. They also found no evidence, including forensic information or geolocation data, that would suggest an adversary had used a form of directed energy such as radio waves or ultrasonic beams,” the Post reports. “In some cases, there was no ‘direct line of sight’ to affected personnel working at U.S. facilities, further casting doubt on the possibility that a hypothetical energy weapon could have been the culprit, one of the officials said.”

The Post’s reporting of this conclusion contradicts an earlier independent study of possible causes. A 2020 study by a committee from the National Academies of Sciences, Engineering and Medicine “felt that many of the distinctive and acute signs, symptoms, and observations reported by [Department of State] employees are consistent with the effects of directed, pulsed radio frequency (RF) energy.”

This is the kind of energy used in less-lethal millimeter-wave weapons like the Active Denial System employed by the US military. This weapon takes a massive amount of power to operate, and it can cause second degree burns on people in the beam’s path if it is held for long enough. The weapon is used to disperse crowds, making the pain felt immediately and in a way that dissipates as people leave the area of the beam. For people who do stay in the beam’s path, spots can become visible on their skin.

[Related: The US military’s heat weapon is real and painful. Here’s what it does.]

The people suffering Havana syndrome symptoms lacked injuries that would match how a microwave weapon works.

“There’s a persistent myth that microwaves heat things from the inside out. Anyone who has heated a frozen dinner knows that this is not true. The outer part of the frozen food thaws first, because it absorbs the microwaves before they can reach the inner part,” wrote Cheryl Rofer, retired Los Alamos National Laboratory chemist, in response to the NAS study. She added: “if a directed microwave beam hit people’s brains, we would expect to see visible effects on the skin and flesh. None of that has accompanied Havana syndrome.”

The new report suggests that, while there may be no single explanation for the symptoms, there are likely other, identifiable causes. One possibility, instead of a weapon causing the harm, was simply the conditions of people in an embassy breathing air passing through clogged ducts, reports the Post. 

Embassy work can be difficult and stressful, to say nothing of the decades when US embassies were regularly violently targeted by insurgencies and terror groups. This includes the US Embassy in South Vietnam in 1965 by the Viet Cong, the 1983 bombing of the US Embassy by Hezbollah in Beirut, Lebanon, and attacks on the US Embassy in Afghanistan in 2011, 2012, and 2019, among others. In light of that history, it can be easy to understand how worsening health might feel like symptoms of an invisible siege. While the report likely rules out known weapons and deliberate attack, it doesn’t negate the fact that people can experience harmful symptoms from sources other than weapons.