LONDON — Scotland Yard detectives on Saturday traced the final steps of a former KGB spy turned Kremlin critic after officials determined he was poisoned by a rare radioactive substance.

A special cabinet council that deals with sensitive diplomatic incidents met for a third day to discuss Alexander Litvinenko's death. A meeting Friday was chaired by Britain's top law enforcement official, home secretary John Reid.

Litvinenko died late Thursday at a London hospital after spending days in intensive care as doctors puzzled over what was causing his organs to fail.

Police said they were not yet treating the case as a murder, rather as an "unexplained death."

Britain's Health Protection Agency said Friday that the radioactive element polonium-210 had been found in his urine, and the police said traces of radiation were found at Litvinenko's home and a ritzy hotel bar and sushi restaurant he visited on Nov. 1, the day he became ill.

In a statement written before he died, Litvinenko called Russian President Vladimir Putin "barbaric and ruthless" and blamed him personally for the poisoning.

Putin responded by accusing his opponents of "political provocation," while calling Litvinenko's death a tragedy.

Russian Ambassador Yury Fedotov was summoned to London's Foreign Office Friday as British diplomats asked Moscow for its assistance investigating the case, government officials said. Putin has pledged to cooperate.

Litvinenko, 43, had told police he believed he had been poisoned on Nov. 1 while investigating the October slaying of Russian journalist Anna Politkovskaya, another of Putin's critic.

Litvinenko worked for the KGB and its successor, the FSB. In 1998, he publicly accused his superiors of ordering him to kill tycoon Boris Berezovsky and spent nine months in jail from 1999 on charges of abuse of office. He was later acquitted and in 2000 sought asylum in Britain.

In Moscow, pro-Kremlin lawmakers pointed the finger at exiled Russian dissidents, claiming the death was part of a plot to discredit the Kremlin.

Russian lawmaker Valery Dyatlenko claimed Friday that Berezovsky, an exile whose asylum in Britain has enraged the Kremlin, may have been involved in the killing, seeking to discredit Putin.

When reached by The Associated Press on Saturday, Berezovsky declined to comment on the accusations leveled by Dyatlenko. In Israel, Russian exile Leonid Nevzlin said Litvinenko's death could be linked to investigations into charges laid against ex-shareholders and former owners of the Yukos oil company.

Nevzlin — a former shareholder in the Yukos oil company — told Israel's daily Haaretz newspaper on Friday that Litvinenko had visited him for a meeting.

He said he had passed on documents related to the campaign of criminal charges and tax claims against Yukos shareholders and officials, including now-jailed founder Mikhail Khodorkovsky. Nevzlin feared the ex-agent's death could be connected to the probe.

Nevzlin was charged by Russian prosecutors with organizing murders, fraud and tax evasion, and lives in self-imposed exile in Israel to evade prosecution.

Polonium-210 occurs naturally and is present in the environment at very low concentrations, but can represent a radiation hazard if ingested.

Pat Troop, chief executive of the Health Protection Agency, said the high level of polonium-210 indicated Litvinenko "would either have to have eaten it, inhaled it or taken it in through a wound."