The British government said Alexander Litvinenko's death was linked to a radioactive substance, polonium-210, in his body. Home Secretary John Reid, the country's top law-and-order official, said experts had been called in to search for "residual radioactive material" at a number of locations.

Litvinenko's statement, read to reporters outside the hospital where the former spy died, accused the Russian leader of having "no respect for life, liberty or any civilized value."

Litvinenko's friend and spokesman Alex Goldfarb said the former spy had dictated the statement before he lost consciousness Tuesday, and signed it in the presence of his wife, Marina.

Putin's government strongly denied involvement.

Putin Friday described the death as a tragedy, but said he saw no proof it had been violent. He said medical documents did not show "that it was a result of violence, this is not a violent death, so there is no ground for speculations of this kind." Kremlin spokesman Dmitry Peskov said allegations of Kremlin involvement "are nothing but nonsense."

"It's so silly and unbelievable that it's not worth comment," said Peskov in Helsinki, Finland, where Putin is at a summit with European Union leaders.

Litvinenko, a vociferous critic of the Russian government, suffered heart failure late Thursday after days in intensive care, London's University College Hospital said.

Doctors said the cause of his illness remained a mystery, discounting an earlier theory that the 43-year-old father of three had been poisoned with the toxic metal thallium or a radioactive substance.

"As part of this investigation, the police have called in expert assistance to search for any residual radioactive material at a number of locations," Reid said.

Friend Andrei Nekrasov, who spoke to Litvinenko just before he lost consciousness, said he had accused Russian intelligence services of poisoning him. Friends said Litvinenko had been on a quest to uncover corruption in Russia's Federal Security Service, or FSB, and unmask the killers of another trenchant critic of the Putin government, journalist Anna Politkovskaya.

"He was completely convinced it was the FSB. There was no doubt in his mind who it was," Nekrasov told Associated Press.

He said Litvinenko had told him: "The bastards got me, but they won't get everybody."

Litvinenko told police that he believed he had been poisoned November 1, while investigating the slaying of Politkovskaya.

His hair fell out, his throat became swollen and his immune and nervous systems were severely damaged.

ASSOCIATED PRESS