A secret Russian intelligence unit is believed to be behind assassination attempts and a failed coup as part of Russian President Vladimir Putin’s efforts to destabilize Europe, according to Western security officials.

An attempt to assassinate a former Russian spy living in Britain and a failed coup in Montenegro were carried out by an elite group inside the Russian intelligence system, known as Unit 29155, the New York Times reported Tuesday.

Unit 29155 has operated in secrecy for at least a decade, but was only recently discovered by Western officials. It’s not known how often the group is deployed, and western intelligence officials warned they don’t know when and where the unit will show up next. The group’s existence has been so covert that even other operatives of the Russian intelligence agency, known as the GRU, were likely unaware of it, according to Western intelligence analysis.

Officers, some of whom are veterans of Russia’s wars in Afghanistan, Chechnya, and Ukraine, travel to and from European countries to carry out their operations.

Western intelligence has linked several operations to the unit. Two officers were involved in a failed coup in Montenegro in 2016 which involved a plot to kill the country’s prime minister and seize the Parliament building. The 2018 poisoning of Sergei Skripal, a former Russian spy, and his daughter in Britain, as well the attempted assassination of Bulgarian arms dealer Emilian Gebrev in 2015 have also been linked to the group.

A retired GRU officer who had knowledge of Unit 29155 told the Times the group has expertise in preparing for “diversionary” missions, “in groups or individually — bombings, murders, anything.”

“They were serious guys who served there,” the retired officer said. “They were officers who worked undercover and as international agents.”