Russia's Investigative Committee said former KGB bodyguard Andrei Lugovoi, who has been accused by British authorities of poisoning Litvinenko with radioactive polonium in London, was also sickened by polonium, The Moscow Times reported.
Lugovoi was poisoned while meeting with Litvinenko in London and has now been "acknowledged a victim in the criminal case," committee spokesman Vladimir Markin said.
The Times noted Lugovoi had repeatedly said in the months after Litvinenko's death that medical tests showed he had not suffered radiation poisoning.
But on Wednesday, Lugovoi told RIA-Novosti. "I am satisfied with this decision because for five years I have been saying that I am a victim, both physically and from a moral point of view."
British authorities have accused Lugovoi of killing Litvinenko, who died after drinking polonium-laced tea, and sickening businessman Dmitry Kovtun during a meeting in a London hotel in November 2006.
Russian investigators say they are now treating Litvinenko's murder and the attempted murders of Kovtun and Lugovoi as one case, with an unknown perpetrator.
Litvinenko, a former KGB agent who became a Kremlin critic, said on his deathbed the Russian government was behind his death.
Russia has refused British demands that Lugovoi, now a member of the State Duma, be extradited to face trial in London, causing tension between the two countries.
Some observers said the Russian investigators' revelation five years after the poisoning may be political, as Logovoi faces re-election Sunday, the Times reported.