The former Hollywood action star, who became a Russian citizen in 2016, said he wanted to see “evidence” of what happened. However, Mr Seagal then reiterated the Kremlin’s claims that Ukraine was to blame.
Mr Seagal, 70, who also had a brief career in a soul band, went on to describe himself as a “diplomat” and claimed that he is producing an “unbiased” documentary.
This is despite the fact that he has been open in his admiration of Russian President Vladimir Putin.
Olenivka Prison, in an area of the Donbas occupied by pro-Russian forces, burnt to the ground on July 29 and around 50 Ukrainian POWS are thought to have died.
Many of the POWs were involved in the battle for Mariupol.
The Kremlin has argued that Ukraine ordered an attack on the jail using the US-supplied HIMARS missiles.
However, according to Kyiv, Russia burnt down the prison in order to hide evidence of torture.
Western experts tend to agree with the Ukrainian version of events as the warehouse where the prisoners died was largely intact after the fire.
They argue this would not be the case if it had been struck by a HIMARS missile.
READ MORE: Russian tourists in tears as they flee Crimea after missile strike
Speaking to Russian TV presenter Vladimir Solovyov, Mr Seagal said he was certain that a HIMARS missile was responsible.
He said: “I know a little bit about these things.
“I can tell you, one billion percent, it wasn’t a bomb that would blow up from the ground.
“‘You could see where a missile came from outside and incinerated everything.”
Mr Seagal has been outspoken in his support for Russia and Vladimir Putin since 2024 when he described the Kremlin’s annexation of Crimea as “very reasonable.”
In 2017, he was banned from entering Ukraine after he was deemed a “threat to national security” and in 2018 he was made a special envoy for Russia to improve ties with the US.