Pulsating electromagnetic energy "convincingly explains" some of the cases of a mysterious illness called "Havana syndrome", a new report by the US intelligence community claims.
There is a bitter controversy over whether the symptoms affecting American personnel around the world are caused by some kind of device or are psychological in nature.
The latest report by a panel of experts from the US intelligence community indicates that the symptoms are "authentic and convincing" and that they may be caused by some external source.
But the panel does not address who might be responsible for it.
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Beginning in Havana, Cuba in 2016, US intelligence officials began to complain of a series of unusual symptoms.
At first, these claims were mostly dismissed within the government.
But over the past year, they have been taken more seriously as US officials have been encouraged to report similar symptoms.
This led to a real avalanche of reports, which numbered at least a thousand cases worldwide.
A CIA study in January of this year found no evidence of a widespread campaign by a foreign state and said many cases could be explained by natural causes or stress.
However, the study acknowledges that a number of cases remain unexplained.
This new study examined more than 1.000 top-secret documents and interviewed witnesses, focusing on a group of people suffering from a specific set of symptoms (officials declined to reveal the exact number).
They concluded that this series of cases could not be explained by medical or environmental conditions and that they could have been caused by some kind of external source or device.
"We learned a lot," said one intelligence official familiar with the panel's work.
The panel found four "main features" that represent the symptoms - including a feeling of pressure and that something is coming from a specific direction or location.
The possibility of five potential causes was investigated.
These are acoustic signals, chemical and biological agents, ionizing radiation, natural and environmental factors, as well as radio frequency and other electromagnetic energy, and it was studied whether the reported symptoms could have been caused by some kind of hidden device.
The panel established that psychological and sociological factors cannot explain the symptoms by themselves, although they could intensify some of the problems in the sufferers.
He also established that they cannot be explained by medical or environmental conditions.
"Several aspects of a unique neurosensory syndrome make it unlikely that they are caused by a functional neurological disorder," the panel concluded.
That pointed to "external stimuli" or a source, according to the panel.
The study showed that pulsating electromagnetic energy, especially in the radio frequency range, "convincingly explains" the main characteristics listed, although it added that there are still some gaps in the information.
It is said that non-standard antennas can produce such effects on the human body.
Such a source can remain hidden and requires only moderate power.
It can also travel through the air and through the walls of buildings.
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A 2021 BBC report and documentary addressed the potential role of such microwaves in causing symptoms of Havana syndrome.
There are no details in the report about what kind of device it might be, or whether the intent was to harm or conduct some kind of surveillance.
The panel did not deal with who could be behind such an activity.
A number of people within the US intelligence community believe that Russia may be responsible and the issue has been raised in meetings, although no definitive evidence has been discovered to support these allegations.
Another plausible explanation for the symptoms, the panel says, is some kind of ultrasound, although it travels more difficultly through buildings, meaning the source would have to be close to the target.
The panel made a number of recommendations, including collecting more standardized data, but some of them, including a section on source detection technology, remain top secret.
In the joint statement of the two highest officials of the American intelligence community, a promise was made that the search for the truth will continue, along with the constant care of the sick.
"We will continue to pursue this, with equal commitment, for as long as necessary," said Director of National Intelligence Avril D. Haynes and CIA Director William J. Burns.
A White House spokesman told the BBC they welcomed the report's findings.
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So how significant is this new report?
This is another major twist in the long-standing mystery of Havana syndrome.
The debate over the cause has been bitter and confrontational so far.
A CIA report last month may have led many to conclude that there may be fewer signs of any hostile activity and that the cases are actually medical or psychological conditions.
That may still be true in the vast majority of cases.
But this panel brings back the possibility of some kind of malicious activity by stating that, at least in a specific subset of cases, pulsed energy or microwaves are a possible cause.
This will inevitably lead to the question of whether it is the result of some kind of surveillance that harms people along the way or whether it is the use of some kind of weapon and, if so, who could be behind it.
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