Washington DC - The US intelligence community has concluded that a series of mysterious health incidents affecting US personnel around the world since 2016 were unlikely to have been caused by a foreign adversary or by some kind of weapon.

Scouring evidence in roughly 1,500 reported cases, seven intelligence agencies unanimously concluded that it was either unlikely or very unlikely that a foreign actor was responsible for the cases, which became known to the public as Havana syndrome after a cluster of cases affected US personnel in Cuba at the end of the Obama administration.

The assessment published Wednesday found evidence "consistently against" the involvement of foreign powers, one intelligence official familiar with the assessment said.

In fact, "what you tend to see among key adversaries is confusion," a second official said.

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Hundreds of intelligence officials generated petabytes of data as part of the investigation, which began with the assumption that a foreign country was targeting US personnel with some kind of radio frequency device. But the agencies assessed that foreign powers do not possess the kind of weapon that would be required to inflict the symptoms experienced by personnel, and could not identify a weapon that could feasibly be responsible for such a wide variety of cases.

It would have required technology with the ability to target individuals in line of sight, sometimes through concrete, without stopping their watches or flickering surveillance camera footage.

Intelligence officials narrowed down the volume of reported cases to those that they determined could identify a source of the problem, the officials said.

In one case, an officer reported symptoms after an unidentified car pulled up beside the officer’s. US intelligence officials tracked down the car and its owner, investigated their family, and followed them for four months.

In others, criminal activity was discovered around a reported case. Drugs and weapons dealers who were in the vicinity were investigated for ties to foreign powers.

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In several cases, faulty HVAC systems audible to most people sent off alarm bells among intelligence personnel who had been told to be on the lookout for signs they were being targeted, leading to reports.

Optical mice paired with computers also surprised intelligence officials for their ability to disrupt frequencies at some of the scenes of reported cases.

Intelligence officials even examined the possibility of extraterrestrial involvement, questioning whether the symptoms could be caused by a device in the sky – "no stone unturned," the first official said.

Early in its investigation, the intelligence community hoped to find a common set of criteria to identify as a syndrome behind the phenomenon. But no set of symptoms emerged.

"The problem is we are not seeing a common set of medical things," the first official said.

Radio frequency and pulsed magnetic energy devices also tend to cause heat on the skin, which was not a consistent symptom among the reported cases.

US officials also asked allies and partners if they had either seen any evidence of a device that could cause the phenomenon, or if their personnel had experienced similar symptoms themselves. "There wasn’t an upswell," the second official said.

Several cases that occurred in the Cuban capital in 2016 that gave the series of anomalous health incidents its moniker, Havana syndrome, remain unexplained.

The second official acknowledged that it was easier to investigate cases in locations where the United States and its partners were "in control of the environment."

The intelligence assessment was careful not to accuse personnel that came forward of succumbing to mass hysteria. But the officials familiar with the report said that environmental factors may have played a role in the spike in cases.

Case reporting has dropped "significantly" in recent months, one official said. While some cases have been reported in 2023, no clusters have been identified.