A police spokesman said: "It is important to stress that we have reached no conclusions as to the means employed, the motive or the identity of those who might be responsible for Mr Litvinenko's death."
Dmitry Kovtun, who met Litvinenko in London on November 1, has also been diagnosed as suffering from radiation poisoning, the Russian prosecutor general's office said.
Kovtun was interviewed by British officers and investigators from the Russian prosecutor-general's office on Wednesday.
Questioning
They are expected to question Andrei Lugovoy, a former KGB spy who was also at the meeting, on Thursday. He has denied any involvement in Litvinenko's death.
"It is important to stress that we have reached no conclusions as to the means employed, the motive or the identity of those who might be responsible for Mr Litvinenko's death."
British police spokesman |
The prosecutor-general has publicly insisted that Russian authorities will carry out any interviews on Russian soil.
Sergei Lavrov, Russia's foreign minister, said: "The Scotland Yard investigation is not affecting the quality of Russia-Britain political relations."
On Monday, Lavrov had expressed concern that ties could be damaged if accusations of high-level Russian involvement continued. The Kremlin has denied any involvement in the death.
Doctors have said that Litvinenko was poisoned with polonium 210, a highly radioactive isotope, which has since been detected in several places around London.
British health authorities said earlier this week that the body could be buried without risk to public health, provided it was in a sealed coffin.
Litvinenko was buried at London's Highgate cemetery - last resting place of Communism's ideological founder, Karl Marx. Litvinenko's coffin arrived with a large white wreath on top.
His wife Marina wore dark glass and several cars with darkened windows entered the cemetery.
Mourners included emigre Russian billionaire Boris Berezovsky, a Putin critic and Litvinenko associate, and Akhmed Zakayev, London envoy for the Chechen separatists with whom Litvinenko had worked.
Chechnya claims
An exiled Chechen leader has said he was convinced that Moscow was behind the death because he had seen similar poisoning cases in the breakaway Russian republic.
Akhmed Zakayev told Sky News: "When the police told me he [Litvinenko] had been poisoned with polonium 210, I immediately associated his symptoms with those people who died in Chechnya.
"They are exactly identical, so I am absolutely convinced that the Russians used polonium on the people of Chechnya."
Zakayev is one of a number of anti-Moscow dissidents living in Britain and has previously accused the Kremlin of having a hand in his friend's death.