Russian opposition leader Alexei Navalny, who died Friday in a remote prison, may have been killed by a single punch to the heart, according to a human rights activist who spoke with the Times of London.

Citing a source at the "Polar Wolf" penal colony where Navalny died, Vladimir Osechkin, founder of the human rights group Gulagu.net, said that the fierce Kremlin critic was found with bruising on his head and chest that would seemingly indicate the use of the "one-punch" technique.

"It is an old method of the KGB's special forces divisions," Osechkin told the Times, referring to the infamous Soviet-era internal security service.

"They trained their operatives to kill a man with one punch in the heart, in the center of the body," he said. "It was a hallmark of the KGB."

Navalny, 47, had been kept outdoors for more than two hours in subzero temperatures in preparation for the fatal blow, Osechkin said, citing the unidentified prison employee.

"I think that they first destroyed his body by keeping him out in the cold for a long time and slowing the blood circulation down to a minimum," Osechkin said. "And then it becomes very easy to kill someone, within seconds, if the operative has some experience in this."

The human rights activist told the Times that former inmates of the Arctic region prison where Navalny died have reported that fellow prisoners were killed by guards in this manner.

Almost a week after Russia's prison service reported that Navalny collapsed and died while on a walk, his official cause of death has not been revealed.

Navalny was Russian President Vladimir Putin's most outspoken domestic adversary, one who had, in 2020, survived a poisoning attempt on his life.

On Monday, Kira Yarmysh, the late dissident's spokesperson, wrote on X that "Navalny's body is hidden to hide traces of the murder. 14-day 'chemical test' is a blatant lie and mockery."

Yarmysh was referring to the Russian authorities' refusal to immediately return Navalny's body to his family, saying it was needed for a "chemical examination."

His mother, Lyudmila Navalnaya, filed a lawsuit Wednesday at a court in the Arctic city of Salekhard over officials' refusal to release her son's body.

Navalny's widow, Yulia, has accused the Kremlin of poisoning her husband with the highly toxic nerve agent Novichok, which was used in his failed assassination attempt.

Osechkin said he didn't think that was the case.

"It's possible, of course, but when someone is under the control of the prison system, there are many options as to how to kill them," he said. "Novichok would leave a trace in his body and would lead directly back to Putin, given he has tried it once before."

Kremlin officials have denied any involvement in Navalny's death.

• Report: Evidence Suggests Navalny Was Poisoned
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