A mysterious illness affecting US diplomats on overseas postings for much of the past decade has been linked to a Russian military intelligence unit by independent outlet The Insider following a year-long investigation.

Carried out in collaboration with CBS flagship news show 60 Minutes and Germany’s Der Spiegel, the investigation concluded that unit 29155 of Russia’s military intelligence (GRU) used sonic weapons and radio waves to target victims, causing a variety of unexplained symptoms known as Havana Syndrome.

Senior members of unit 29155, a group one former CIA officer said was responsible for “conducting lethal operations and acts of sabotage”, apparently received awards and promotions for their development of “non-lethal acoustic weapons”. Other members were geolocated to places “just before or at the time of reported anomalous health incidents”.

Moscow has previously denied any connection to Havana Syndrome, and even US intelligence concluded that foreign involvement was “very unlikely”, but offered no alternative explanation for the condition.

The syndrome was named after the Cuban capital where its first case was recorded in 2016, though The Insider investigation suggested that other attacks may have occurred as early as 2014, when American officials stationed in Germany reported experiencing similar symptoms.

To date more than 100 American diplomats and intelligence operatives have reported experiencing symptoms of Havana Syndrome, which includes chronic headaches, vertigo, tinnitus, insomnia, nausea, hearing loss, and dizziness.