STEVE INSKEEP, host:
British authorities are following the radiation as they investigate the poisoning of a Russian man. Traces of radiation lead back to Russia from London where former spy, Alexander Litvinenko, died.
And that's where the investigation is moving as NPR's Gregory Feifer reports from Moscow.
GREGORY FEIFER: Four British counterterrorism officers arrived in Moscow yesterday. One of the key potential witnesses they hope to question is Andrei Lugovoi, a former Russian intelligence officer who met Litvinenko in a London hotel on November 1st, the day he fell ill.
Lugovoi denies involvement, telling Russian television he's ready to fully cooperate with the investigators.
Mr. ANDREI LUGOVOI (Former Russian Intelligence Officer): (Through translator) I contacted Scotland Yard, myself. I said I was waiting for them in Moscow. I'm not even considering returning to London because of the hysteria the Western press has whipped up around this case.
FEIFER: Litvinenko, a vocal Kremlin critic, died from ingesting radioactive polonium 210 last month. In a deathbed statement, he blamed Russian president Vladimir Putin for ordering his death. Russian authorities deny the charge. Putin said the poisoning was a provocation by his critics, seeking to discredit Russia. And Foreign Minister Sergei Lavrov, on Monday, warned London not to politicize Litvinenko's death.
Mr. SERGEI LAVROV (Foreign Minister, Russia): (Speaking Foreign Language)
FEIFER: I told Foreign Secretary Margaret Beckett the only thing we're concerned about, he said, is to avoid speculation about this question. Litvinenko's friends are urging the British investigators to speak to a jailed Russian intelligence officer, who's accused the Russian Security Service - the FSP - of creating a hit squad to kill Russian émigrés in Britain.
But Western experts say it's difficult to believe Putin would have sanctioned the killing. Gerard Burke, a former assistant director of the U.S. National Security Agency, said it's imperative for Putin to cooperate with the investigation.
Mr. GERARD BURKE (Former Assistant Director, U.S. National Security Agency): The thing that would maybe tend to neutralize the damage that's already been done by this bad publicity, is to say, you know, I'm going to personally direct this investigation or have it directed under my personal aegis; and I'm going to assist the British government, note - to the full extent on my capacities, and really go after this thing.
FEIFER: The British investigators have their work cut out for them. Most believe elements within the FSP top the suspect list, because only state agencies have access to polonium.
Former CIA officer, Jack Platt, says current FSP members or Rogue former officers may have killed Litvinenko to intimidate Moscow's critics overseas. He says the operation was expensive and well-planned, but that the execution was completely botched.
Mr. JACK PLATT (Former CIA Officer): They could not have predicted that here would lie a man, who's slowly but surely dying, and yet he has an opportunity to be interviewed by police and detectives - I'm told, for up to 20 hours. Don't we all realize that dying testimony carries great weight in any society?
FEIFER: The Russian media have publicized a wide array of theories. Some believed Kremlin hardliners wanted to trap Putin into making a formal break with the West. Others say criminal groups acted to stop Litvinenko's investigations of their activities, or even that Litvinenko accidentally contaminated himself while smuggling polonium.
But what scientists saying the radioactive isotope can be traced to the reactor in which it was produced, some hope there's a chance Litvinenko's death can be explained.
Gregory Feifer, NPR News, Moscow.
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