Russian President Vladimir Putin offered condolences Thursday as the Pentagon assessed that the leader of the Wagner Group mercenary front that marched his forces on Moscow earlier this summer was “likely” killed in a plane crash.

Yevgeny Prigozhin, who led the aborted revolt in June, was listed among 10 names on the private plane’s passenger list, according to a report from Tass, Russia’s state news service, which cites the country’s aviation authority. All 10 people aboard the flight were killed.

In his first statement about the incident, Putin expressed condolences to the families of the victims, saying “it is always a tragedy.”

“I've known Prigozhin for a very long time, since the early 1990s,” Putin said in televised remarks, according to a translation by Tass. “He was a man of a no easy fate. He made some serious mistakes in his life, but he also achieved the needed results – both for himself and, when I asked him, for the common cause. The way it was in recent months.”

Still, official confirmation that Prigozhin was among the dead was slow in coming. Meanwhile, theories began circulating about what led the plane to fall from the sky, like an internal explosion or a missile launched at the plane.

“Our initial assessment is that it’s likely Prigozhin was killed,” Pentagon spokesman Gen. Pat Ryder said. “We’re continuing to assess the situation. We don’t have any information to indicate right now – the press reporting stating that there was some type of surface-to-air missile that took down the plane – we assess that information to be inaccurate.”

A day earlier, President Joe Biden said what many were likely thinking when asked about the reports.

“I don’t know for a fact what happened, but I’m not surprised,” Biden said.

Asked if he thought it was the work of Russian President Vladimir Putin – whose enemies regularly fall victim to fatal accidents or mysterious attacks – Biden responded, “There’s not much that happens in Russia that Putin’s not behind. But I don’t know enough to know the answer.”

Prigozhin – who had seemed to largely disappear from public view after he was permitted under a deal brokered by Belarusian President Alexander Lukashenko to take refuge in his country in the wake of the mutiny – appeared to maintain a precarious position within Putin’s inner circle in the weeks before the plane crash. He seemingly was allowed to remain at the head of his forces and secure billions of dollars in government contracts despite his aborted rebellion against Moscow in June, when a column of some 25,000 Wagner troops loyal to him conducted a stunning about-face and marched along with fighting vehicles and heavy weaponry toward the capital before calling off the advance.