(CNN) - A U.S.-based friend of a poisoned Russian ex-spy says he has given police the name of a suspect he believes orchestrated the killing of Alexander Litvinenko.
"The truth is, we have an act of international terrorism on our hands. I happen to believe I know who is behind the death of my friend Sasha (Litvinenko) and the reason for his murder," Yuri Shvets said in an exclusive interview with The Associated Press by telephone from the United States, reports Trend.
Shvets, also a former Russian intelligence officer, said he had known Litvinenko, who died in London after he was exposed to a rare radioactive element, since 2002 and had spoken to him on November 23, the day he died.
He told AP he was questioned by Scotland Yard officers and an FBI agent in Washington last week. A police official in London confirmed officers had interviewed Shvets.
British police investigating the poisoning death of Litvinenko, 43, have officially requested assistance from their Russian counterparts and are due in Moscow as early as Monday to collect information.
The Russian embassy in London issued visas to the Scotland Yard officers Sunday.
Nine British detectives will speak to witnesses who met the former Russian spy in London shortly before his death. These include Andrei Lugovoi, another former spy who met Litvinenko on November 1 -- the day he fell ill.
Lugovoi, also a former Russian agent, agrees he and businessman Dmitry Kovtun met Litvinenko that day at London's Millennium hotel. But Lugovoi, now back in Moscow, says they discussed a business opportunity and he denies anything to do with an attempt on Litvinenko's life.
Litvinenko, a former agent of the Russian state security service, died in London after poisoning with radioactive polonium 210.
From his deathbed, Litvinenko accused Russian President Vladimir Putin of ordering his slow and agonizing death, which sparked a health scare in Britain and strained London's relations with Moscow.
The Kremlin denies involvement and has promised full cooperation with the British investigation. Other theories have centered on the possible involvement of rogue Russian agents.
British Home Secretary John Reid reassured fellow EU ministers Monday that the health threat from polonium-210 is "absolutely minimal."
He updated his European counterparts on the latest developments in the inquiry during talks in Brussels -- part of the UK government's policy of widening co-operation as the investigation goes on, the UK's Press Association reported.
Litvinenko's death led to widespread fears that others could have also been contaminated after police grounded three British Airways planes for testing for radioactive substances. All three planes were cleared over the weekend.
Doctors treating Litvinenko's friend, Italian academic Mario Scaramella, said Sunday that he continued to show no symptoms of poisoning. But London's University College Hospital said that further tests were being carried out on Scaramella, who also met Litvinenko at the Itsu sushi bar in Piccadilly, London, on November 1.
In an interview with Italy's RAI TG1 evening television news Scaramella said doctors had told him that his body contains five times the dose of polonium-210 considered deadly. "So my mood isn't the best," he told the channel.
Also among those British police may want to see in Russia is former intelligence officer Mikhail Trepashkin, who is now serving a four-year sentence in an Urals prison for divulging state secrets.
In letters smuggled out of Russia last week Trepashkin claimed that a secret hit squad had been set up to target Litvinenko and others in 2002 and offered himself as a witness in the British investigation.
Litvinenko's friend Alex Goldfarb told Reuters: "Mr Trepashkin has substantive information that might be of interest to investigators and his lawyers are prepared to facilitate contact with him.
He said Trepashkin was in poor health, adding "this could be the only opportunity that the British authorities have to speak to him."
Goldfarb also paid tribute to Litvinenko's wife Marina, who has herself tested positive for a low level of radiation.
"She is a very strong woman, I am actually surprised how well she has coped," he told PA.
Mrs. Litvinenko and her son 12-year-old Anatole, were joined by close friends including Litvinenko's friend and sponsor, Russian tycoon Boris Berezovsky and Chechen rebel envoy Akhmed Zakayev for an emotional meal on Friday night to mark the ninth day since the spy's death. Within the Russian Orthodox tradition the ninth day after a death is a significant milestone.