US diplomats who suffered from the mysterious “Havana syndrome” may have been targeted by Russian sonic weaponry, according to a new joint media investigation.

The condition first came to light in 2016 when US diplomatic staff working at the embassy in Cuba’s capital, Havana, reported suffering symptoms such as headaches, ringing in the ears, and cognitive dysfunction.

Similar ailments were also reported by embassy staff in China, Europe and the US capital, Washington DC.

Many theories have been put forward about the cause, including the possibility of a hostile state using a type of directed energy weapon involving soundwaves.

However, the CIA previously ruled out the possibility of foreign involvement in the vast majority of cases and suggested stress as a major factor.

Meanwhile, some studies have put forward the mysterious phenomena of mass psychogenic illness – otherwise known as mass hysteria – as the probable cause.

However, a new joint report by The Insider, Der Spiegel and CBS’s 60 Minutes, has suggested a shadowy Russian intelligence unit may be involved.

They say the year-long investigation has “uncovered evidence” suggesting the health incidents known as Havana syndrome “may have their origin in the use of directed energy weapons wielded by members of (the Russian GRU) Unit 29155”.

Moscow immediately dismissed the allegations as “groundless” on Monday.

Asked about the report, Kremlin spokesperson Dmitry Peksov said: “This is not a new topic at all; for many years the topic of the so-called ‘Havana syndrome’ has been exaggerated in the press, and from the very beginning it was linked to accusations against the Russian side.

“But no one has ever published or expressed any convincing evidence of these unfounded accusations anywhere.

“Therefore, all this is nothing more than baseless, unfounded accusations by the media.”

At the heart of the investigation is Unit 29155, a shadowy Russian intelligence unit tasked with carrying out activities to destabilise European countries, as well as foreign assassinations.

The group is thought to have been behind the attempted assassination of Sergei Skripal in the Salisbury poisonings of 2020.

As part of the investigation, The Insider, a Russia-focused investigative media group based in Riga, Latvia, reported that members of Unit 29155 had been placed at the scene of reported health incidents involving US personnel.

They also reported that senior members of Unit 29155 received awards and promotions for work related to the development of “non-lethal acoustic weapons”.

The issue first began in late 2016, when US diplomats and spies serving in Cuba began reporting bizarre sounds and sensations followed by unexplained illnesses and symptoms, including hearing and vision loss, memory and balance problems, headaches and nausea.

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Over the years, many hundreds of US officials have come forward to report suspected incidents in more than a dozen countries.

In 2018, NBC reported that US intelligence officials considered Russia a leading suspect in what some of them assessed to have been deliberate attacks on diplomats and CIA officers overseas.

The National Academies of Sciences, Engineering and Medicine said in a report in 2020 that some of the observed brain injuries were consistent with the effects of directed microwave energy.

Then, in 2022, the CIA released an interim assessment, which concluded that the syndrome was not the result of “a sustained global campaign by a hostile power” and ruled out foreign involvement in 976 of the 1,000 cases it reviewed.

In the remaining cases, the agency could not rule out foreign involvement.

An unclassified version of a report compiled by seven US intelligence agencies, published in March last year, added that a foreign adversary’s involvement was “very unlikely”.

Other studies have put forward the theory of mass psychogenic illness, a phenomenon where symptoms spread through a population without a contagion.

In these studies, it has been suggested that the stress of working overseas in hostile or unfriendly countries, where the job often involves anxiety about possible surveillance, could have been a factor.