The so-called Havana syndrome could have been caused by pulsed electromagnetic energy, according to a declassified US intelligence report published Wednesday.

The syndrome was first reported in 2016, when US and Canadian embassy staff in Cuba reported unusual symptoms, including pain, ringing ears and other issues. Since then, embassies across the world have reported being affected, sparking diplomatic incidents.

To date, no evidence has been found to prove any attack or perpetrator. The CIA last month cast doubt on the syndrome being caused by foreign attacks.

What did the intelligence report say?

A joint investigation by the CIA and the Director of National Intelligence found at least some of the incidents could not be explained by psychosocial factors alone.

Some two dozen cases out of a total of 1,000 that have been reported are yet to be explained.

It said the signs and symptoms from the anomalous health incidents (AHIs) were "genuine and compelling."

"Some incidents have affected multiple persons in the same space, and clinical samples from a few affected individuals have shown early, transient elevations in biomarkers suggestive of cellular injury to the nervous system," the report said.

The two agencies said they were "making progress" in a joint statement. 

Havana syndrome's combination of symptoms "cannot be easily explained by known environment or medical conditions," the report added. 

The redacted report found: "pulsed electromagnetic energy, particularly in the radiofrequency range, plausibly explains the core characteristics" of the syndrome.

It said such a device could be concealed, could penetrate walls, and could travel long distances.

What is Havana syndrome?

While there clearly is no standardized definition of the affliction, those who have been affected have reported drowsiness, fatigue, headaches and problems with hearing and vision.

Cases were first reported in Cuba, hence the name, among US and Canadian diplomats and their families in 2016.

But reports have since popped up in Colombia, Vietnam, Russia, China, Austria and even Berlin.

Medical experts who looked at some of those first affected in Cuba found concussion-like symptoms, except that they didn't disappear.

ab, aw/sms (AP, Reuters)