By Lucy Papachristou

LONDON (Reuters) -A 73-year-old American jailed by Russia as a mercenary for Ukraine protested his innocence when his U.S.-based legal team and family finally tracked him down in April, months after he vanished into the vast Russian prison system, they said.

Stephen Hubbard, a retired schoolteacher, was sentenced last October to almost seven years in a penal colony after a court found him guilty of serving in a Ukrainian territorial defence unit against Russian forces, tasked with manning a checkpoint.

Russian state media reported that he had entered a guilty plea in the closed-door trial.

The U.S. Embassy in Moscow has not been granted consular access to Hubbard, a State Department spokesperson said, adding that U.S. officials have requested his immediate release.

Martin De Luca, his U.S.-based lawyer, told Reuters it was not until this April that his legal team learned Hubbard was being held in a facility in the Mordovia region, east of Moscow.

"The first thing Hubbard wanted to talk about when he was able to make contact with the outside world was: 'It's not true,'" said De Luca, who made his first public comments on the case to the New York Times this week.

"They (Russian soldiers) grabbed him from his house. He was not in any combat or military unit", De Luca recalled Hubbard saying.

Joseph Coleman, a son from Hubbard's first marriage who lives in Cyprus, said he spoke to his father in prison by phone for less than five minutes on May 28.

"He did sound a little down," Coleman told Reuters. "He said, 'I'm tired of being a slave.'"

At least eight other Americans are currently imprisoned in Russia, which has stepped up arrests of alleged mercenaries for Ukraine since its 2022 invasion of its neighbour.

But Hubbard is the only one designated by the U.S. as "wrongfully detained," making him a top candidate to be returned in any future prisoner exchange. The Kremlin said last month the two sides were discussing a possible swap involving nine people on each side.

A document written on the letterhead of the IK-12 penal colony, signed by a prison official and seen by Reuters, says that Hubbard is incarcerated there.

Russia's federal prison service did not respond to an emailed request for confirmation from Reuters. Other U.S. citizens previously jailed in Russia have been incarcerated in the same region.

VIDEO CLUES

Hubbard, a Michigan native who taught English abroad for decades, had moved to Izium in eastern Ukraine in 2014 to be with a Ukrainian girlfriend, but by 2022 he was living there alone, his family said. Russian forces captured Izium in April 2022.

After his arrest, his family struggled to establish what had happened to him.

They caught glimpses of him in videos posted online in pro-war Russian Telegram channels. One showed what appeared to be a staged interrogation.

In another, Hubbard appeared with his hands zip-tied and whimpered as a man slapped him with a plastic sandal. His sister, Patricia Hubbard Fox, identified her brother in both videos in conversations with Reuters. The agency could not verify when and where the videos were taken.

"He is so non-military," Hubbard Fox told Reuters last year, expressing doubt that her brother would have taken up arms for any state.

"He never had a gun, owned a gun, done any of that... He's more of a pacifist."

TRACKING HIM DOWN

After Hubbard's trial, De Luca and his team at a U.S. law firm began working to secure his release.

They picked up the case in late February. It wasn't easy to find him, De Luca said.

"Russia is still a functioning country. There are laws, bureaucracies, processes that get followed," he said.

The team located Hubbard at the penal colony in Molochnitsa, a very small town about a seven-hour drive from Moscow.

De Luca said the team has been able to call Hubbard three times since April. He described him as weak after months living in a prisoner-of-war camp.

(Reporting by Lucy Papachristou; Editing by Philippa Fletcher)