The British government says former Russian spy Alexander Litvinenko was poisoned by radiation.
In London Friday, the country's Health Protection Agency said a large quantity of the radioactive element polonium 210 was found in Litvinenko's urine.
They said it is not yet clear how the material entered his body.
Police investigating the case say they have found radioactive material in at least two locations that Litvinenko was known to have been before he became sick.
One is a sushi bar where he had dinner with an Italian contact November first.
The other is a London hotel where he met two Russians on the same day. Litvinenko died in a London hospital Thursday after a three-week fight for his life.
In a statement read following his death, the former K.G.B. agent accused Russian President Vladimir Putin of being responsible for his death.
He called Mr. Putin "barbaric and ruthless," with "no respect for life, liberty or any civilized value."
In Helsinki Friday for an EU summit, Mr. Putin described Litvinenko's death as a tragedy, but he said there is no reason for speculation that Mr. Putin had any role in the incident.
The former Russian agent had been investigating the recent killing in Moscow of Russian journalist Anna Politkovskaya, a frequent critic of the Kremlin.
Litvinenko defected to Britain six years ago after accusing Russia's Federal Security Service - the successor to the Soviet K.G.B. - of ordering the killing of Russian tycoon Boris Berezovsky.
He also contended that Kremlin agents had a major role in bombings that killed about 300 Russian civilians in 1999 and sparked retaliatory attacks on Chechen separatists. Moscow has repeatedly denied any involvement in his illness.
Information for this report is provided by AP and REUTERS.