The plane carrying Prigozhin seemed fine on radar until a 'sudden downward' drop in its last 30 seconds

• A plane crash in Russia on Wednesday killed 10 people on board. Yevgeny Prigozhin is presumed dead.
• It plummeted suddenly after appearing fine on radar, a flight-tracking expert told Reuters.
• Some have speculated that air defenses shot down the plane.

The plane carrying Yevgeny Prigozhin, the mercenary who rebelled against Vladimir Putin, showed no signs of trouble on radar before a steep drop in the last 30 seconds of its flight, Reuters reported.

The Embraer Legacy 600 jet that crashed in Russia on Wednesday, claiming the lives of all 10 passengers, apparently with the former Wagner mercenary chief onboard, was traveling from Moscow to St. Petersburg.

Prigozhin was named as one of the passengers, and is presumed dead.

It downed near the village of Kuzhenkino in the Tver Oblast, Russian officials said.

Ian Petchenik of Flightradar24, which tracks flight data, told Reuters that everything looked normal with the plane until 6.19 p.m. local time when the aircraft made a "sudden downward vertical" motion.

The plane plummeted more than 8,000 feet from its cruising altitude of 28,000 within approximately 30 seconds, he added.

"Whatever happened, happened quickly," Petchenik said, according to the news agency.

Video footage of the fall, shared by several media outlets, shows the plane in a nosedive with a trail of smoke behind it.

Petchenik told Reuters that there was "no indication that there was anything wrong with this aircraft" prior to its dramatic drop.

The Financial Times noted that the plume trailing the plane resembled shots fired from anti-aircraft defenses.

This theory was echoed in a post on Grey Zone, a Telegram channel linked to the Wagner group, which alleged that Russia's anti-aircraft defenses had shot down the plane, the newspaper reported.

The social media channel also claimed that residents near the crash heard "two bursts of characteristic air defense fire" before the crash, per the Financial Times.

According to Reuters, unnamed Russian media sources also backed the notion that the plane had been shot down by one or more surface-to-air missiles, though the news agency was unable to confirm that.

Another theory is that some kind of bomb was on the plane. John Sawers, the former head of Britain's MI6 spy agency, told the BBC Thursday that he thought "there was some device on board that brought the plane down suddenly."

Russian investigators have opened a probe into the incident. President Joe Biden suggested on Wednesday that Russian President Vladimir Putin was likely behind the crash, though he didn't speak with certainty.

"You may recall, I was asked about this," Biden told CNN. "I said I would be careful what I rode in. I don't know for a fact what happened, but I'm not surprised."

UK defense sources also told the BBC that the plane was most likely brought down by the Russian government, placing the blame on Russia's Federal Security Service (FSB).

"Likely to strengthen position of Prigozhin's enemies: Shoigu and Gerasimov," said the BBC's Security Correspondent Frank Gardner in a tweet.