A former KGB agent who says he warned Kremlin foe Alexander Litvinenko before his fatal poisoning that he was being hunted by government assassins was released from prison on Friday.

Mikhail Trepashkin, who investigated allegations that the security service was involved in a series of 1999 apartment house bombings, was arrested in October 2003, days before he was scheduled to testify about the bombings, which authorities blamed on Chechen separatists.

The bombings were among the justifications cited by Russia for resuming full-scale war in Chechnya, but some critics alleged they were staged by the Federal Security Service, the KGB's main successor agency known by its initials FSB.

Litvinenko, also a former security service agent, fled to Britain in 2000 and co-wrote a book blaming the security service for the bombings. He was killed in London last year with a dose of the rare radioactive isotope polonium-210.

Trepashkin, 50, walked out of the prison in Nizhny Tagil about 900 miles east of Moscow and traveled in a small van to the regional capital Yekaterinburg on Friday, where he spoke to a small crowd of supporters and journalists at the roadside.

"It's the Kremlin that did this," he said of Litvinenko's death. "They didn't forgive him for exposing that the FSB had a department involved in extrajudicial killings."

Trepashkin was arrested on charges of illegal weapons possession and convicted of divulging state secrets.

"The worst is in the past. Before, I fought on my own and now I have many more supporters," he said Friday, appearing energetic. "I've served four years for things I haven't done."

Trepashkin also said that an FSB colonel met with him in August 2002 and asked him to join a group targeting Boris Berezovsky — a self-exiled Russian tycoon living in London — and Litvinenko.

Trepashkin said he refused to cooperate with the team, whose task was to "mop up" Berezovsky, Litvinenko and their accomplices.

He said earlier that he had warned Litvinenko himself of the alleged death squad.

From prison, Trepashkin wrote that FSB officers possessed poisons that could be applied to a car's steering wheel, door handles, telephone receivers and elsewhere — some of which would not leave a trace in the victim's body.

Russian prosecutors have dismissed Trepashkin's allegations, and refused to allow British investigators to talk to him when they visited Russia last year as part of an investigation into Litvinenko's death.

Britain has called for the extradition of another former security agent, Andrei Lugovoi, to face trial in the Litvinenko killing, but Russia has rejected the request.

Many Russian politicians have suggested that Litvinenko was killed by opponents of President Vladimir Putin in order to cast aspersions on the Kremlin.

As a secret service investigator, in the 1990s Trepashkin got two state awards for breaking up criminal gangs and seizing their arms caches. He also has a three-year record of serving on Soviet nuclear submarines.

Trepashkin left the FSB in 1996. He claimed he was removed in connection with a corruption investigation.

He said Friday he would fight for his conviction to be thrown out and would continue his activity as a lawyer and human rights defender, including exposing what he called "horrific" abuses in places of detention.

"In prisons, they have revived (Soviet dictator Joseph) Stalin's practice of creating extrajudicial, tougher conditions for certain prisoners on orders from above."