Moscow undertook its own inquiry into the November 2006 murder of the defector and Kremlin critic, dismissing British investigators' evidence against their chief suspect, Russian national Andrei Lugovoi, as ungrounded and refusing his extradition.
"The first phase of the investigation has shown that the polonium has no identification signs," Alexander Bastrykin, head of the investigative committee at the Russian Prosecutor General's Office, said in a newspaper interview.
"We are trying to determine the polonium's original source, which is very important," Bastrykin said.
Scotland Yard said Litvinenko had received a fatal dose of polonium November 1, when he met with Lugovoi, former Kremlin borderguard-turned businessman, and his business partner Dmitry Kovtun at a luxury hotel. Kovtun has been treated as a witness in the case.
Russia also said its Constitution did not permit the extradition of its nationals, and suggested trying Lugovoi at home if sufficient evidence was provided.
The millionaire businessman, who owns a private security firm, Lugovoi plans to run for the lower house of parliament in the December elections. Membership in the State Duma would give him immunity from prosecution under Russian law.
Lugovoi earlier said the Crown Prosecution Service's accusations were a lie inspired by the British leadership and secret services.
The extradition dispute has strained relations between Russia and Britain, which sparked a tit-for-tat row involving expulsions of diplomats and visa restrictions.