One Havana night in 2016, a CIA officer woke with a loud piercing sound in his ear and a splitting headache. No one else could hear the ringing but the pressure in his skull seemed to be coming from somewhere, as if an invisible beam of energy was following him around the room.

It was the start of a slew of mysterious cases, now dubbed Havana syndrome, that have befallen Western diplomats and spies around the world, from embassies in China and Russia to the streets of Washington, a hotel room in Australia, even the lawns of the White House. Possible causes range from pesticides and “mass hysteria” to covert sonic or microwave weapons, and now US officials are investigating another suspected case in Vietnam that has delayed US Vice-President Kamala Harris’s visit.

What’s going on – and is it as much like a John le Carré thriller as it sounds?

What is Havana Syndrome?

That’s the million-dollar question. Officially, US government agencies list it as “unexplained health incidents” and doctors still call it a medical mystery. Symptoms generally hit all at once, some people describe a loud ringing or a buzzing in their ears, and intense pressure in their head. They may get nausea, dizziness, insomnia, vision impairment or vertigo. In one 2019 case reported by GQ, a White House staffer fell ill while walking her dog in Washington after passing a parked van. Her dog started seizing up, then she felt it too. When she left, the symptoms went away.

Many of those affected around the world have been in the middle of sensitive intelligence operations, often for the CIA, and also found the worst of their illness went away if they moved locations. In 2019, two officials accompanying then US president Donald Trump during a visit to the United Kingdom fell ill with suspected cases. US diplomats in Germany have been hit by the syndrome in recent months as have about two dozen US officials in Vienna who were said to be working on Russia-related issues.

Brain scans of the dozens of US and Canadian officials who first reported the syndrome in Cuba in 2016 revealed brain damage, the kind consistent with a bomb blast or car crash but with no sign of force. It was as if they had a concussion without the concussion, one specialist said, an “immaculate concussion”. Some of the 200-odd people now believed to have been affected worldwide have had to retire due to ongoing neurological problems, including “brain fog”.

What could be behind it?

The Americans suspect some kind of covert sonar or microwave weapon is to blame for the mystery incidents, which are often called attacks. Such weapons exist, says UNSW defence technology expert Dr Jai Galliott, formerly of the Australian Navy. They fall into a class of technology already used by militaries around the world that harness focused beams of energy such as noise or microwaves, he says.

We don’t see sound with our human eyes but its waves can affect the world around us (think of an opera singer shattering a wine glass). “Sonic weapons can kill,” says Galliott, citing research that has found sound above 210 decibels can damage the lungs and lead to serious complications such as an air embolism, as well as impacting the nervous system. “But it’d be a highly impractical killer,” he says. “Much easier to drug or shoot someone.”

That’s because, generally, sound has to be very, very loud to do damage in one go – say, a jet plane landing right beside you. Hardly covert, as it was in Havana. And embassies are already hardened with soundproofing to keep away eavesdroppers. Plus, “sound loses its strength in the air”, Galliott says, so a weapon would also need to be close, with a narrow, focused beam.

According to a major report by America’s Academy of Sciences in 2020, microwave beams (or “pulsed radiofrequency energy”) are the most likely culprit as they can affect brain function without causing gross structural damage, explaining the symptoms. (Another report in the Journal of the American Medical Association the year before came to a similar conclusion.) Powerful microwaves can also make people hear strange, clicking noises that aren’t there, known as the Frey effect, as the microwave beam creates pressure and tiny vibrations in your head. Much more covert.

During the Cold War, Russia blasted microwaves at the US embassy in Moscow for years. The US didn’t report anyone getting sick back then, but experts say Russian research into highly concentrated microwaves has now advanced considerably.

So, it’s microwaves?

This theory has its sceptics, too. Microwave beams are already used (harmlessly), by police to capture the speed of cars, for example. But a microwave weapon capable of causing brain damage hasn’t been seen before, leading some scientists to question the verdict of the American academies’ expert panel. Some have pointed instead to pesticide poisoning (which the report ruled out as there was no evidence of exposure and the clinical facts didn’t align properly).

Others have suggested it’s psychosomatic, what they call “mass hysteria”, where high-pressure stress leads to real symptoms that people can “catch” from one another (this was again dismissed by the scientific panel and other doctors as many of the cases were not aware of others falling ill).

The FBI also initially found that “mass delusion” could explain the events in Havana, which infuriated victims, who were not interviewed by investigators directly. The agency is now reported to be reassessing that conclusion in light of new cases.

Galliott says microwave weapons have emerged in warfare more recently than sonic ones – mostly as high-energy laser beams designed to shoot down drones and missiles. But he says it’s possible that a device shooting “pulsed radiofrequency energy” could be behind the mystery. He points to “The Thing” of Cold War Soviet legend, a wireless microphone once concealed in a splendid wooden carving gifted to the US ambassador to Moscow in 1945. The bug was activated by a strong electromagnetic signal beamed in from outside.

This time, the incidents “could be some technology used as a modern equivalent [to the Thing], designed to garner information”, with the illness an unintended consequence on the people nearby, he says. Or it “could be purposely designed for psychological effect – or entirely made up by the CIA. Russia is a scientifically advanced nation and certainly capable of developing such technology but so is China and a whole bunch of other nations.”

If it is an attack, who’s attacking?

After the Havana cases first emerged in late 2016, the Trump administration pulled out more than half its embassy staff there. The Cuban government denied any involvement, though some suspected that local hardliners may have been trying to disrupt newly restored diplomatic ties with the US.

The Kremlin has also denied they are behind the mystery.But US intelligence has reportedly used mobile phone data to place Russian intelligence officers in close proximity to many of the cases since (including those of two CIA officers struck down with the syndrome while in hotels in Australia and then Taiwan).

Meanwhile, Germany’s Der Spiegel magazine claims to have seen evidence that Russia has been working on sending out dangerous radio waves through a target’s mobile phone. If true, that could explain why some officials have seemed to suffer fresh bouts of the illness even after returning to the US.

Former national security official Dr William Stoltz, now a senior adviser at the ANU’s National Security College, says US agencies have signalled that they believe Russia’s military intelligence service, the GRU, is behind the incidents. “The GRU, they’re the cowboys of Russian intelligence,” he says. “They’re well-resourced and the most likely to do these kinds of high-risk activities. Russia has a long history of experimenting with these kinds of nefarious methods of attack, assassinations with umbrellas and the like.”

But finding new creative ways to spy on and sabotage enemies is not unique to Russia. As Stoltz notes, the US has its own history of experimenting with psychological warfare. “Think of the mind control experiments the CIA did with LSD,” he says. “Israeli intelligence, too, have a reputation for being innovative, but they’re not as theatrical as Russia. [Putin critic] Alexei Navalny was poisoned through his underpants. Some of [their methods of sabotage] are so over-the-top and complex, it’s almost comical. But it’s designed to create fear ... to throw sand in the gears.”

Still, the growing list of countries in which the syndrome is appearing makes Stoltz wonder if the Russians have sold or shared the technology with other nations, too, if that is what’s at work here. “In Vienna, there were a lot of incidents surrounding Iranian nuclear negotiations there. Now, in Vietnam, we are seeing it potentially target Vice-President Harris’s delegation at a critical time for geopolitics in Asia and that country” where both China and the US are vying for influence. (Harris delayed but did not cancel her trip as investigations into possible cases of the syndrome in Hanoi continue. But China’s top diplomat used the opening to swoop in and pledge 2 million vaccine doses for Vietnam). “It makes me think perhaps the Chinese and the Iranians have got their hands on this tech too and are looking to disrupt things themselves,” Stoltz says.

But what would be the point of such attacks?

Even if there is some ongoing medical damage from the syndrome, it’s hardly a sniper bullet, Stoltz says. “It can seem like it’s just a nuisance. But that does fit with the Russian [GRU] MO. They are the wreckers, the agents of chaos.”

Politics and diplomacy are inherently human activities, he says, relying on people meeting, on careful negotiation, and skilled staff. “So by creating a device or even stories of a device like this that could sow anxiety among the intelligence community and diplomats, it could disrupt important negotiations. And if people are getting horrific headaches for days or weeks on end, you could undermine the ability of an entire embassy office to do their job properly.”

Is it changing diplomacy?

Diplomats are generally afforded protection in foreign lands, although they are the regular target of surveillance and, sometimes, sabotage. Stoltz says the attacks, if that is what they are, seem quite brazen. Embassy staff have been recalled over many of the cases so far. After the original Havana incidents, the US also issued a travel warning saying its diplomats had experienced “targeted attacks” and expelled Cuban diplomats from Washington. Responses have since softened somewhat. When US officials were evacuated following cases at their Guangzhou consulate in China in 2018, then secretary of state Mike Pompeo said it seemed to be the same syndrome as Cuba but labelled them “health incidents”, not attacks.

Still, if the incidents continue, causing perhaps ongoing harm to embassy staff, Stoltz says they could risk escalation in a number of ways. “Someone like the US could decide to treat it as a physical attack on their diplomats. Even targeting the Vice-President’s visit does start to get pretty close to the edge of that line.”

Or there could be some tit-for-tat use of similar technologies. “But the West has been pretty disciplined in not stooping to Russia’s level when it comes to things like biochemical weapons [and poisonings]. I sometimes think [Russia] would like nothing more than to drag us down into the mud with them.”

Still, Galliott doesn’t think the consequences so far have strayed far outside the usual back-and-forth of “the game”, as espionage and sabotage among nation states is called. Hostilities between the US, China and Russia are already high, he says, and “diplomats”, often themselves spies, are trained to handle a range of threats including the biological.

“Let’s just say that they probably have bigger worries.”