On the tarmac of a Moscow airport in late August, Yevgeny Prigozhin waited on his Embraer Legacy 600 for a safety check to finish before it could take off. The mercenary army chief was headed home to St. Petersburg with nine others onboard. Through the delay, no one inside the cabin noticed the small explosive device slipped under the wing.

When the jet finally left, it climbed for about 30 minutes to 28,000 feet, before the wing blew apart, sending the aircraft spiraling to the ground. All 10 people were killed, including Prigozhin, the owner of the Wagner paramilitary group.

The assassination of the warlord was two months in the making and approved by Russian President Vladimir Putin’s oldest ally and confidant, an ex-spy named Nikolai Patrushev, according to Western intelligence officials and a former Russian intelligence officer. The role of Patrushev as the driver of the plan to kill Prigozhin hasn’t been previously reported.