Traces of radiation had been found at Litvinenko's north London house, the sushi restaurant where he met a contact Nov. 1 and a hotel he visited earlier that day, Clarke said. The restaurant and part of the hotel were closed, with officers removing materials in heavy metal boxes.

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Clarke said extensive tests by forensic toxicologists on behalf of police - which began before Litvinenko's death - had on Friday confirmed the presence of polonium-210.

"There is no risk to the public unless they came into close contact with the men or their meals," said Katherine Lewis, a spokeswoman for the Health Protection Agency.

Experts said small amounts of polonium-210 - but not enough to kill someone - are used legitimately in Britain and elsewhere for industrial purposes.

Professor Dudley Goodhead, a radiation expert at the Medical Research Council, said that "to poison someone, much larger amounts are required and this would have to be manmade, perhaps from a particle accelerator or a nuclear reactor."

Chris Lloyd, a British radiation protection adviser, said it would be relatively easy to smuggle polonium into the country, because its alpha radiation would not set off radiation detectors.

Doctors treating Litvinenko had said Thursday that they could not explain his rapid decline. They discounted earlier theories that the father of three had been poisoned with the toxic metal thallium.

Lewis, the Health Protection Agency spokeswoman, said doctors had not discovered the presence of polonium-210 in Litvinenko earlier because hospitals do not normally test for the alpha-ray radiation it emits.

University College Hospital, where Litvinenko died, said Friday it could not comment further because the case was being investigated by police.

Litvinenko's friends had little doubt about who was to blame.

They said the former spy, who sought asylum in Britain in 2000 and became a citizen, worked tirelessly to uncover corruption in Russia's Federal Security Service, the successor agency to the Soviet-era KGB, and unmask Politkovskaya's killers.

Litvinenko had worked for the KGB and then the Federal Security Service until he publicly accused his superiors in 1998 of ordering him to kill Russian tycoon Boris Berezovsky. He spent nine months in jail on charges of abuse of office, but was later acquitted and moved to Britain.

In Moscow, pro-Kremlin legislators pointed at Berezovsky, who amassed a fortune in dubious privatization deals after the 1991 Soviet collapse but fled to London after falling out of favor with Putin. He has been a persistent critic of Putin and worked with Litvinenko.

Lawmakers questioned whether the two critics had a falling out and argued the Kremlin had nothing to gain from Litvinenko's death. "I think this is another game of some kind by Berezovsky," Valery Dyatlenko said on Channel One.

"It was an excruciating death and he was taking it as a real man," Walter Litvinenko told reporters outside the hospital, his voice choked with emotion.

Goldfarb said the attack bore "all the hallmarks of a very professional, sophisticated and specialist operation."

Another friend, Andrei Nekrasov, said Litvinenko told him: "The bastards got me, but they won't get everybody."

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