British detectives investigating the poisoning of ex-spy Alexander Litvinenko have asked to return to Moscow, according to reports.
Alexander Litvinenko was a former Russian security officer
Russia's top prosecutor was quoted as saying Scotland Yard wants to carry out further inquiries in the city.
Mr Litvinenko died on November 23 in a London hospital.
His body contained radioactive polonium-210 and nine UK police officers visited Russia last month to carry out interviews.
According to the Itar-Tass news agency, Prosecutor General Yuri Chaika said: "Literally yesterday, a new international investigative request from Great Britain arrived containing a request seeking permission for a group of British investigators probing the Litvinenko case to come to Moscow.
"I do not rule out that after the trip by our representatives to London, we will again receive our colleagues here."
Mr Litvinenko, a former Federal Security Service (FSB) officer, met three other former Russian secret agents just before he fell ill in London - Dmitry Kovtun, Andrei Lugovoi and Vyacheslav Sokolenko. They denied any involvement in his poisoning.
His friends have accused the Kremlin of ordering his assassination in response to his criticism of President Vladimir Putin. The Russian government has rejected the claims.
Mr Chaika confirmed that Russian investigators wanted to speak to two Russian exiles in London: Businessman Boris Berezovsky and former Chechen rebel leader Akhmed Zakayev, both of whom have been given asylum in the UK.
Mr Chaika said: "Concerning the Litvinenko case itself, we are also interested in solving this crime, since, first, he was a Russian citizen, and second, we have our own versions of the murder.
"This murder may have been committed by Russian citizens living abroad."
According to Alex Goldfarb, a friend of Mr Litvinenko's, Mr Berezovsky will agree to be interviewed by Russian investigators so long as his safety is guaranteed by British authorities.
Mr Goldfarb said Mr Berezovsky had expressed fears that he could be poisoned with polonium during the course of any questioning.
Mr Goldfarb said: "He would speak to Russian prosecutors provided that the British authorities ask him to do so, provided that his security is guaranteed.
"You can expect anything from these people. That's why he got asylum in the first place."
Mr Chaika said on Sunday that the Russian investigators would leave shortly for London.
British police were subject to strict conditions during their first trip to Russia in December. They were not allowed to question witnesses directly, and officials stressed that Britain would not be able to extradite any Russian suspects.
Mr Chaika's deputy, Alexander Zvyagintsev, was quoted by state-owned Rossiskaya Gazeta last week as saying that Russian authorities wanted to question more than 100 witnesses in the UK.
Since Mr Litvinenko's death, traces of the polonium-210 have been discovered in several restaurants and hotels.
A spokesman for Scotland Yard said: "We are not prepared to comment at this time."