There also remains widespread scepticism in the global intelligence and security community.
If you had developed a top-secret science-fiction death ray, why give up the advantage of secrecy for the sake of intimidating a few diplomats?
The Russians, pointed out one sceptical British security expert who asked to remain anonymous, have plenty of tried and tested techniques for doing that.
Then there is the inconsistency.
Other than several Canadian government employees sharing a compound with US staff in Cuba, there are no reported incidents of the syndrome affecting any other country’s employees.
Why would the Russian secret services, assuming they are behind the attacks, spare close US allies, including the UK, who they have targeted relentlessly in the past?
The Telegraph asked the Foreign Commonwealth and Development office if British diplomats had been affected. It said it was not aware of any.
But it is the method that has confounded many experts.
“It is the science that is throwing me completely. It could be done, but it would be really difficult. Really, really difficult,” said Philip Ingram, a former senior British military intelligence officer who has studied the weaponisation of directed energy.
Cold War-era research established that targeted microwaves can induce individuals to hear sounds inside their heads, a phenomenon that matches some cases of Havana syndrome.
The Soviet Union and the United States both experimented with the potential military applications, but never worked out how to effectively weaponise it.
However, Russia, the US, and China are also known to have restarted such research in more recent decades.
In 2003, the US Navy even awarded just under $100,000 to a Californian firm called Waveband to build a “breadboard prototype” of a weapon exploiting exactly that phenomenon.
The “Medusa” device was described in a summary report written the following year as a non-lethal perimeter-protection weapon designed to deter trespassers, but cause them no lasting damage.
The requirements laid out in the document – portable, low power, and able to switch from crowd to individual coverage – would also fit a more offensive hypothetical Havana Syndrome weapon.
There is no evidence the United States ever built more than a very basic prototype. The existence of Russian or Chinese equivalents remains speculative.