- Former MI6 chief John Sawers suggested a bomb could have been on board Prigozhin's plane.
- Several theories exist as to how the plane crashed, killing all those on board.
- "I would have thought there was some device on board," Sawers told the BBC.
A former MI6 chief said on Thursday that Yevgeny Prigozhin's private jet could have harbored an on-board "device" that caused it to crash.
Prigozhin, the leader of the Wagner Group is presumed dead after being named among 10 people on the plane's list of passengers. Russia's civil aviation authority said that none survived the crash on Wednesday, northwest of Moscow.
Many questions remain about how the Embraer-135 plummeted from the skies that day, and several theories exist as to who may have been behind it — President Vladimir Putin himself being an obvious candidate. As of Thursday morning, the Kremlin and the Russian defense ministry were silent about the incident.
UK defense sources told the BBC that Russia's domestic intelligence agency, the FSB, was most likely to have carried it out — but exactly how is an open question.
Speaking about it on BBC radio's "Today," former MI6 chief John Sawers suggested a device on-board could have been the culprit.
Traces of an air-defense missile — one of the theories being floated — would be "detectable through satellite means," Sawers said. He said current MI6 officials may have that information, though not former ones like him.
He continued: "But it looks as though this was - I'm not an expert on these things, but I would have thought there was some device on board that brought the plane down suddenly and killed all those on board."
Sawers didn't specify the device he was referring to, but an obvious candidate would be on-board explosives.
Former air-crash investigator Anthony Brickhouse told Insider's Azmi Haroun that a crashed plane bears multiple clues that can help point to the cause of its demise.
Investigators began trawling the site for clues on Thursday, Reuters reported.
Sawers was doubtful that Prigozhin could be alive. However, he noted that "you can never fully be certain of facts in places like Russia," he said, adding that all indications suggest Putin is behind it.
"If there is a slim chance that [Prigozhin] is not dead, that he wasn't on that plane, he will be soon," he said.
Noting that figures like Dmitry Utkin, a key figure in Prigozhin's orbit, was also on board, he said: "It's a way of taking out the entire Wagner leadership all in one go."
