"One of the core versions could be that Litvinenko carelessly handled polonium which he may have had. His real hatred for those in power in Russia then, for the intelligence service, for everything Russian should be taken into account," Andrei Lugovoi, now a Russian lower house MP, told journalists.
Litvinenko died of radioactive poisoning on November 23, 2006. British investigators accused agent-turned-businessman Lugovoi of the murder, and demanded his extradition, sparking a major diplomatic row.
Large traces of radioactive polonium-210 were found in Litvinenko's body, but British authorities have not yet made public any official document specifying the exact cause of his death or the results of the autopsy.
Russia has long demanded that the U.K. provide additional evidence to back up its accusations against Lugovoi. However, the newly-appointed British ambassador to Russia, Anne Pringle, said recently that "London has already submitted sufficient evidence to extradite him to Britain."
Lugovoi has repeatedly denied involvement in the murder.
Litvinenko was fired from the FSB (formerly the KGB) following a 1998 press conference in which he and a number of other FSB officers alleged that they had been ordered to murder and kidnap a number of high-profile figures.
He received British citizenship in 2006 and published two books in the U.K. alleging the involvement of the Russian security services in a series of apartment bombings in Russia in 1999.