The FBI has made the phantom "Havana Syndrome" — that saw embassy staff feel ill after hearing high-pitched chirping — its number one priority.
The federal police and counter-intelligence agency said some 200 overseas-based diplomatic staff and their families have experienced "anomalous health incidents".
The FBI told news agency Reuters the issue was "a top priority for the FBI, as the protection, health and well-being of our employees and colleagues across the federal government is paramount."
The unexplained set of symptoms was first reported in early 2017 by US and Canadian embassy staff in the Cuban capital, just after then-president Donald Trump took office and a year and a half after the US and Cuba restored diplomatic relations after a 54-year hiatus.
Up to 26 staff said they had felt ill after hearing a high-pitched or grating sound, with the first incidents in December 2016.
The symptoms included pain in one or both ears, a feeling of pressure or vibrations in the head, tinnitus, visual problems, vertigo, nausea, cognitive difficulties, fatigue and dizziness.
One theory was that foreign agents could allegedly be using a science-fiction 'sonic weapon' to attack and incapacitate US diplomats and spies.
But an official report for US State Department two months ago concluded that the most likely cause was the chirping of the Indies short-tailed cricket.
The reports neatly coincided with the then-new Trump administration's policy of rolling back some of the limited détente with Cuba initiated by Barack Obama's government.
Apart from the Canadians reportedly affected in Havana, the syndrome mysteriously only strikes US officials and their relatives.
On Thursday CIA spy chief William Burns attempted to lay blame for the phantom ailment with Russia’s Federal Security Service (FSB) and Foreign Intelligence Service (SVR), without presenting evidence.