Wagner boss Yevgeny Prigozhin was on board a plane that crashed north of Moscow killing all 10 passengers on board, Russia's Civil Aviation Authority has said.
There has been no comment from the Kremlin or Russia's defence ministry as Prigozhin's death is yet to be officially confirmed.
The crash comes two months after the Wagner mercenary boss led a short-lived munity against Russia's military top brass.
Prigozhin was described at the time by Russian President Vladimir Putin as a "traitor".
The mercenary boss used to be Mr Putin's chef and was formerly a close confidant of the Russian leader.
Mr Putin has spoken publicly for the first time since the crash after delivering a speech at the BRICS summit - but he did not mention Prigozhin.
The fact the Wagner boss's death has not been confirmed and an absence of verifiable facts has led to speculation following the crash.
'Plane brought down by bomb in a wine crate'
Many observers, and some of the Wagner boss's supporters, have said the Russian state is responsible.
It is suggested Mr Putin may have wanted to down the plane in revenge for the mutiny.
US President Joe Biden is among those who believes the Russian leader was involved.
He said: "I don't know for a fact what happened but I'm not surprised... There's not much that happens in Russia that Putin's not behind."
Ukrainian presidential adviser Mykhailo Podolyak said Mr Putin had been "waiting for the moment" to take his revenge.
Mr Podolyak added that the Russian leader's alleged "elimination of Prigozhin" was a message to Russia's elites that "disloyalty" equals death.
One rumour circulating is that the plane was downed by a bomb in a crate of wine that was gifted to the passengers on the flight.
The claim is unverified.
Former British intelligence officer Christopher Steele told Sky News: "Certainly it's an inside job, the suggestion is that it's a bomb in a wine crate.
"That's a kind of ironic end for Putin's former caterer."
Footage suggests plane may been brought down by missile
Sky's military analyst Sean Bell has said footage of the plane descending to the ground with a wing missing suggests the crash was not the result of a bomb.
He said: "Generally speaking, when there is a bomb inside an aircraft it blows out, and therefore probably wouldn't take a wing off.
"Whereas a surface-to-air missile system, or an air-to-air missile system, generally tries to seek out the juiciest, meatiest part of an aircraft, and that is where the wing spar joins the main aircraft.
"That would explain why it could rip a wing off, and soon as it did that, the fate of the aircraft was sealed."
Janis Sarts, director of NATO's strategic communications centre of excellence, told Latvian television "the downing of the plane was certainly no mere coincidence".
The crash came the same day that Russian media reported that General Sergei Surovikin, a former top commander in Ukraine who was reportedly linked to Mr Prigozhin, was dismissed from his post as commander of Russia's air force.
It comes after some unnamed sources told Russian media they believed the plane had been shot down by one or more surface-to-air missiles.
Whoever or whatever was behind the crash, Prigozhin's death would rid Mr Putin of someone who had mounted the most serious challenge to the Russian leader's authority since he came to power in 1999.
Others who have opposed Mr Putin or his interests have also died under unclear circumstances or come close to death, including outspoken political leaders and journalists.
The Kremlin has always denied any state involvement in such incidents.
Criminal case opened
Rosaviatsia, Russia's aviation agency, has published the names of all 10 people on board the downed plane.
This included Prigozhin and his deputy Dmitry Utkin who helped found the Wagner mercenary group.
The Investigative Committee of Russia has said it is looking into the crash which happened while the plane was travelling from Moscow to St Petersburg.
The committee has confirmed in a statement that 10 people were killed and a criminal case has been opened - but there was no mention of Prigozhin.
A Reuters reporter at the crash site has seen men carrying black body bags away from the scene on a stretcher.
Part of the plane's tail and other fragments lay on the ground near a wooded area where forensic investigators had erected a tent.
Grey Zone, a Telegram channel linked to Wagner, has pronounced their leader dead.
The group also hailed Prigozhin as a hero and a patriot who had died at the hands of unidentified people it called "traitors to Russia".
Mourners left flowers and lit candles near Wagner's offices in St Petersburg early on Thursday.
The plane that crashed, a Brazilian Embraer Legacy 600 model, has only recorded one accident in over 20 years of service, according to website International Aviation HQ, and it was not due to mechanical failure.
Embraer said it has complied with international sanctions imposed on Russia and had not provided maintenance for the aircraft since 2019.
The plane showed no sign of a problem until a precipitous drop in its final 30 seconds, according to flight-tracking data.
Prigozhin was last seen in a video on Monday that appeared to have been shot in Africa, where he spoke about making the continent "free".
The Wagner Group boss released footage of himself speaking while wearing camouflage and holding a rifle.