Russia launched its own investigation after rejecting Britain's request to extradite Lugovoi for Litvinenko's death in London in November 2006, the Russian information agency Novosti reported Tuesday.
Thus far, Russian forensic experts have been unable to trace the polonium that poisoned Litvinenko, said Alexander Bastrykin, chief investigator with the Russian Prosecutor General's Office.
"We are trying to determine the polonium's original source, which is very important," Bastrykin said.
Scotland Yard maintains Litvinenko received a fatal dose of polonium when he met with Lugovoi, a former Kremlin borderguard, at a hotel in London.
Lugovoi, now a millionaire businessman, is seeking a seat in Russia's lower house of parliament, the Duma, which would give him immunity from prosecution.