What is Havana syndrome?
Like something out of a spy thriller, a US State Department report last December concluded that the "most plausible mechanism" for the mysterious systems of Havana syndrome was "direct, pulsed radiofrequency energy" – a type of radiation also found to emanate from microwaves.
Previously affected government staffers, including CIA operatives posted overseas in Cuba and China, reported waking in the middle of the night experiencing "severe pain" including intense pressure, a piercing sound in one ear with "directional features", disequilibrium and nausea.
Victims also suffered headaches, memory and hearing loss, and difficulty sleeping for years – some even becoming wheelchair-bound or forced to wear weighted vests to correct their balance.
And while the State Department report didn't suggest any particular country was behind the attacks, Russia has previously been accused of using a secret "microwave weapon" to target CIA agents.
Senior CIA official Marc Polymeropoulous told GQ in October he felt "like I was going to both throw up and pass out at the same time" when he was woken in his Moscow hotel room in late 2017.
According to the New York Times, there have been more than 130 cases worldwide since 2016, with at least one episode taking place overseas in the last two weeks.
Biden administration increases efforts to determine cause
Facing pressure from other politicians and victims, US President Joe Biden and his administration are dialling up efforts to "identify the cause of these incidents, determine attribution, increase collection efforts, and prevent" what the intelligence community now deems "anomalous health incidents", a spokesman for the Office of the Director of National Intelligence told CNN in a statement.
Some politicians in the US have also expressed concern over the way matters were handled by the CIA, accusing the agency of failing to take proper care of its victims.
"I'm appalled that many of these individuals who were injured in the line of duty have had to fight to get adequate medical care, to have their injuries even recognised and acknowledged and to receive financial compensation," Republican Senator Susan Collins, a member of the Senate Intelligence Committee, said.
The committee's chairman, Mark Warner, said an investigation into the syndrome must be "better co-ordinated".
"I think there's a level of seriousness given to this now that frankly was not there until [CIA] Director Bill Burns came and made this a priority," the Virginia Democrat said.
An official with direct knowledge of the incidents told the network it "took a while for certain people to take it very serious" – noting that during the Trump administration, those who reported experiences with Havana syndrome weren't always believed.
"I don't believe that we as a government, in general, have acted quickly enough," former Marine and Arizona Democrat Ruben Gallego told the New York Times.
"We really need to fully understand where this is coming from, what the targeting methods are and what we can do to stop them."