Washington DC - The mysterious so-called Havana Syndrome symptoms experienced by US diplomats in recent years have been linked to a Russian intelligence unit, according to a joint media investigation released Monday.

Havana Syndrome was first reported in 2016 when US diplomats in Cuba's capital reported falling ill and hearing piercing sounds at night, sparking speculation of an attack by a foreign entity using an unspecified sonar weapon.

Other symptoms including bloody noses, headaches, and vision problems were later reported by embassy staff in China, Europe, and Washington DC.

The diplomats may have been targeted by Russian sonic weaponry, according to the joint report by The Insider, Der Spiegel, and CBS' 60 Minutes.

Russia Evan Gershkovich's family speaks out on one-year anniversary of Russian arrest

The year-long investigation "uncovered evidence suggesting that unexplained anomalous health incidents, also known as Havana Syndrome, may have their origin in the use of directed energy weapons wielded by members of (the Russian GRU) Unit 29155," the report said.

Russia's 29155 unit is responsible for foreign operations and has been blamed for several international incidents, including the attempted poisoning of defector Sergei Skripal in the UK in 2018.

Moscow dismissed the allegations as "groundless" on Monday.

"This topic has been talked up in the press for many years already. And from the very beginning most often it's linked to the Russian side," Kremlin spokesperson Dmitry Peskov told a news conference.

"But nobody has ever published any convincing evidence, so all this is nothing more than a groundless and unfounded accusation," he said.

Washington closed its Havana immigration office in 2018 under a US policy shift toward Cuba and also in response to fears at the time that the Havana Syndrome" was a result of a microwave or other electronic attack.

US intelligence also said in 2022 that intense directed energy from an external source could have caused some cases of Havana Syndrome, officially known as anomalous health incidents (AHIs).

But intelligence agencies concluded in March 2023 that "there is no credible evidence that a foreign adversary has a weapon or collection device that is causing AHIs."

Russia Russia detains defense minister over corruption suspicions in rare move

Washington announced the reopening of its immigration office in Havana in August 2023.

The joint investigation suggests the first cases of Havana Syndrome may have occurred in Germany two years earlier than the cases reported in Havana in 2016 that gave the syndrome its name.

"There were likely attacks two years earlier in Frankfurt, Germany, when a US government employee stationed at the consulate there was knocked unconscious by something akin to a strong energy beam," the report said.

The New Yorker reported in July 2021 that about two dozen US intelligence officers, diplomats, and other government officials in Austria had reported problems similar to the Havana Syndrome since President Joe Biden took office the same year.

The United States deployed medical and scientific experts to study the alleged attacks and those affected have been extensively examined to try to understand their afflictions.