Anti-terrorist police investigating the death by radioactive poison of a former Soviet spy probed theories ranging from a Kremlin plot to a case of self-poisoning as health authorities prepared Sunday to test scores of people for traces of radiation.

Opposition politicians, meanwhile, pressed for a British government statement on Alexander Litvinenko's death by poisoning, which officials have called "unprecedented."

"It is essential that other dissidents living in Britain are reassured about their safety and there are also questions about how polonium-210 came to be used in Britain," said David Davis, the Conservative law-and-order spokesman.

Litvinenko - a former KGB agent who was a fierce critic of Russian President Vladimir Putin - died Thursday of heart failure after falling gravely ill from what doctors said was poisoning by the radioactive element polonium-210.

Litvinenko, 43, told police he believed he was poisoned Nov. 1 while investigating the October slaying of Russian journalist Anna Politkovskaya, another critic of Putin's government. He was moved to intensive care last week after his hair fell out, his throat became swollen and his immune and nervous systems suffered severe damage.

Britain's Health Protection Agency called the poisoning by polonium-210 - a rare radioactive element usually produced in a nuclear reactor or particle accelerator - "an unprecedented event."

The discovery triggered meetings of the government's top-level emergency committee, COBRA, and an attempt to trace everyone who came into contact with Litvinenko on the day he fell ill.

Traces of radiation were found at Litvinenko's north London house, a sushi bar where he met a contact Nov. 1 and a hotel he visited earlier that day, police said.

The Health Protection Agency said 300 people who had visited the locations have contacted officials. They will be screened for radiation, the agency said.

"We are clearly taking this extremely seriously," the agency's director of radiation, chemicals and environmental hazards, Roger Cox, told Sky News television. "There is a lot of radioactivity involved."

But the agency insisted the risk to others was low because polonium-210 can only contaminate if it is ingested, inhaled or taken in through a wound.

Detectives have finished their search of the sushi bar where Litvinenko met an Italian contact, Mario Scaramella, on Nov. 1. The building was being decontaminated, police said.

Litvinenko's contaminated body was released to a coroner late Saturday, and government pathologists were awaiting advice on whether it was safe to perform an autopsy.

In a dramatic statement dictated from his hospital bed and read outside the hospital after his death, Litvinenko accused the "barbaric and ruthless" Putin of ordering his poisoning.

Putin has called the death a tragedy and denied involvement.

On Sunday, Alexander Lebedev, a former KGB spy who is a member of Russian parliament, said Putin's government played no part in the death.

"I completely rule out the possibility of that being done on official orders from anyone in the authorities," Lebedev told Sky News.

London Metropolitan Police said they were investigating it as an "unexplained death," rather than a murder. They have not ruled out the possibility that Litvinenko may have poisoned himself.

Litvinenko and his allies, however, were adamant that the Russian intelligence services were to blame.

The dead spy's friend, Andrei Nekrasov, said Sunday he doubted the order to kill Litvinenko had come directly from Putin.

"I think Mr. Putin's orders on this are unlikely," he told British Broadcasting Corp. television. He said it was more likely the work of rogue elements in the security forces.

The Sunday Times newspaper reported that as he lay dying, Litvinenko named an alleged Russian agent he feared had been sent to hunt him down, and said he previously complained to police that the man had harassed him at home.

Litvinenko claimed the Russian agent was not directly involved in his poisoning but had been sent to monitor his activities, the newspaper said.

Metropolitan Police said it could not immediately confirm whether officers would seek to interview the alleged Russian agent.

Britain's Foreign Office has asked Moscow for help with the investigation but the government has avoiding criticizing the Kremlin or suggesting official involvement in Litvinenko's death.

On Sunday, however, Northern Ireland Secretary Peter Hain accused Putin of presiding over "huge attacks on individual liberty and on democracy."

Hain said Putin's tenure had been "clouded" by incidents "including an extremely murky murder of a senior Russian journalist" - Politkovskaya.

In interviews given before his death, Litvinenko claimed that as a Russian agent, he was ordered to hire assassins to kill rivals to Kremlin-favored business leaders and execute whistle-blowers who threatened to expose corruption.

Litvinenko was recruited into the Soviet-era KGB and worked for its successor, the FSB, where he was promoted to a specialist counterterrorism and organized crime unit.

Litvinenko publicly accused his superiors in 1998 of ordering him to kill Russian tycoon Boris Berezovsky, who was living in exile in London.

He spent nine months in jail from 1999 on charges of abuse of office. Litvinenko was later acquitted and moved to Britain, which granted him asylum in 2000. He had recently become a British citizen.

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