"Mister President, today there is no greater stain on the Russian state's reputation than the murder of my husband. You have the opportunity to uncover before the world the secret of this crime, name those who ordered it and the perpetrators and close this dark page," Marina Litvinenko said in a statement made public on the day of Dmitry Medvedev's inauguration.
A large amount of radioactive polonium-210 was found in the Russian security service defector's body, but the British authorities have not yet made public any official document specifying the exact cause of his death or the results of the autopsy.
Litvinenko was fired from the FSB (formerly the KGB) following a 1998 press conference in which he and a number of other FSB officers alleged that they had been ordered to murder and kidnap a number of high-profile figures.
London has requested the extradition of its chief suspect in the Litvinenko case, Russian businessman and MP Andrei Lugovoi. Moscow has refused to extradite the former Kremlin security guard, saying its Constitution does not permit this.
Lugovoi, who met with Litvinenko in London before the ex-FSB officer fell ill, denies any involvement and says Litvinenko tried to recruit him for the British Intelligence Service (MI6).
The dispute has led to a dramatic deterioration in relations between London and Moscow, including tit-for-tat expulsions of diplomats and the closure of two British Council offices in Russia.
Litvinenko received British citizenship in 2006 and published two books in the U.K. alleging the involvement of the Russian security services in a series of apartment bombings in Russia in 1999. He also wrote about the issue in Russia for the Novaya Gazeta newspaper, where investigative journalist Anna Politkovskaya worked at the time of her murder in 2006.