Litvinenko, a Kremlin critic who lived in exile in London, died in a London hospital on November 23 after receiving a lethal dose of radioactive polonium-210.
Andrei Lugovoi, a Russian businessman, has been named by the British media as a key suspect.
Speaking to the BBC on January 31, Lugovoi denied his involvement in Litvinenko's death.
"When I hear any official accusations, I will be happy to answer. As far as I am concerned, I am still a witness, not a suspect," Lugovoi said.
The Russian Prosecutor-General's Office has said that it will not extradite Lugovoi if he is charged in the murder.
The investigation has focused on the Millennium Hotel in London where Lugovoi and a second suspect, Dmitry Kovtun, met with Litvinenko on November 1.
Polonium-210 was found at the hotel and investigators think that the radioactive substance was concealed in some food or drink.
Shortly after the hotel meeting Litvinenko fell ill.
(compiled from agency reports)