IN 2016, a group of American diplomats in Cuba began experiencing the same eerie symptoms.

Seemingly out of nowhere, their hearing went awry. They could hear loud noises with no logical source, developed headaches and nausea, and fell gravely ill.

Medical professionals were at a loss to explain the phenomenon, with theories ranging from collective delusion to a highly advanced attack from enemy forces.

But now — almost two years after the symptoms first presented themselves — experts say they may have narrowed down the cause.

If they’re right, this was definitely no coincidence.

MYSTERY ILLNESS STRIKES DIPLOMATS

In late 2016, a mysterious illness began plaguing American diplomats stationed in Cuba.

A total of 24 US personnel and family members began experiencing hearing loss, traumatic brain injury and severe headaches at roughly the same time.

It started with auditory hallucinations. More than 80 per cent of the afflicted said they could hear high-pitched sounds and noises coming directly from a specific section of a room, before they fell ill with nausea and dizziness.

Some could no longer remember words. Others had difficulty maintaining their balance. A few described an intense sensation of pressure in their ears, similar to that of being in a moving car with the windows open on a windy day.

The symptoms later came to include vertigo, insomnia and cognitive impairment.

One diplomat told the Associated Press he woke up to a “blaring, grinding noise” in his Havana hotel. Others said they heard a “loud ringing or a high-pitch chirping similar to crickets or cicadas” in quick bursts at night, and that mysterious blaring noises could be heard from certain locations.

“It was annoying to the point where you had to go in the house and close all the windows and doors and turn up the TV,” an affected diplomat told ProPublica. “But I never particularly worried about it. I figured, ‘I’m in a strange country, and the insects here make loud noises.’”

A few nights later, he and his wife invited over the family of a fellow official staying next door. The same noise sounded from the yard - and his neighbour heard it too.

“I’m pretty sure those are cicadas,” the first diplomat said.

“Those are not cicadas,” his neighbour replied. “Cicadas don’t sound like that. It’s too mechanical-sounding.”

One day, another embassy colleague told the diplomat he was leaving Cuba straight away. He said medical specialists had diagnosed him with a series of serious problems, including hearing loss, that had plagued him for months.

The diplomat went for examination, and he — along with 22 other Americans and eight Canadians — was diagnosed with a range of eerie and unexplainable symptoms, including headaches, nausea and hearing loss.

Doctors said they were exhibiting symptoms of concussion.

The sudden illnesses renewed tensions between the US and Cuba, but despite years of research and speculation, medical professionals were unable to reach a conclusion as to what caused it.

In June, the State Department issued a health alert for China after similar incidents unfolded in the southern city of Guangzhou.

WHAT CAUSED THE MASS ILLNESS?

For more than a year, medical professionals studied what might have been causing these drastic symptoms to appear.

After three months, 86 per cent of the group had vision issues, 81 per cent had cognitive issues, 71 per cent struggled with balance and 70 per cent still reported hearing problems and headaches.

Brain researchers at the University of Pennsylvania were unable to shed light on the mysterious cause, which the US Department of State deemed a “health attack”.

Tensions increased between the US and Cuba over the incident, with US officials deeming a “specific attack” and chasisting the Cuban government for failing to protect its diplomats.

Only eight months earlier, then-president Barack Obama had commemorated the two countries’ renewed relationship.

The State Department pulled most of its personnel out of the country, expelling a number of Cuban diplomats from the US. Then-Secretary of State Rex Tillerson suggested the Cuban government was to blame, saying: “I still believe that the Cuban government, someone within the Cuban government, can bring this to an end.”

Cuba, outraged by the accusations, denied knowledge of any attack on those affected, but agreed to work with the US to determine the cause.

Some Cuban and US scientists said the group was suffering from mass psychogenic illness — a form of collective delusion.

Others dismissed this, saying the symptoms were consistent with some form of brain trauma — only the symptoms persisted longer than most mild concussions usually do.

In a study, the Pennsylvania University scientists said: “These individuals appeared to have sustained injury to widespread brain networks without an associated history of head trauma.”

But many US officials were convinced that some Americans, at least, had been targeted by a sophisticated political enemy.

State Department medical director Dr Charles Rosenfarb said it was “most likely related to trauma from a non-natural source”.

“No cause has been ruled out. But the findings suggest this was not an episode of mass hysteria.”

WAS RUSSIA BEHIND THE AILMENTS?

Russia was long considered a suspect for the illensses, with intelligence analysts saying they had both the motive and means to carry out attacks on American diplomats.

Investigators could never find substantial evidence to suggest Russia was responsible for the symptoms.

But according to a new NBC News report, US intelligence agencies now see Moscow as the main suspect.

Evidence from communications intercepts has pointed to Moscow’s involvement during the investigation involving the FBI, CIA and other agencies, NBC reported, citing three unidentified US officials and two other people briefed on the probe.

They believe the ailments may have been triggered by electromagnetic waves or microwave weapons.

The Cuban government also remains under suspicion, with the report noting Russian intelligence agencies operate in the country.

A senior administration official told CNN investigators have torn apart buildings searching for the source of the sounds. No acoustic devices were found, leading authorities to believe the injuries were based on microwaves beamed from a nearby location, with the “sounds” merely masking the microwave attacks.

According to the New York Times, the weapon itself would be portable, and could be held in one’s hand or mounted on a vehicle, with high-powered versions capable of firing beams up to several kilometres away.

Many governments, including the US and Russia, have studied ways to weaponise microwave weapons.

The evidence, however, is still not conclusive enough for the US to assign blame publicly to Moscow.

The Kremlin is yet to respond to the report, and the Cuban government continues to deny involvement.