Wagner boss Yevgeny Prigozhin was on a plane that crashed in western Russia, according to the country's aviation authority.
His name was on the list of those "declared as travelling on board" the plane, along with key Wagner commander Dmitry Utkin.
The Russian Emergencies Ministry said that private aircraft "came down” near the village of Kuzhenkino - killing all on board.
The ministry says the jet was flying from Moscow to St Petersburg, and that 10 people were on board.
Ten bodies have been found at the crash site, according to state media.
Prigozhin has been keeping a low public profile since heading his short-lived mutiny in June, which lasted only 24 hours.
He last appeared in a video earlier this week which was said to have been filmed in an unspecified location in Africa.
The Wagner mercenary group was founded in 2014 and was highly active in the Ukraine war until the June mutiny.
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