Prime Minister Tony Blair said Tuesday that police were determined to find out who was responsible for the radiation death of a former Russian spy, as five more people were sent to a clinic to be tested for radioactive contamination.

One of the last people to see the dead man before he fell ill was being held under tight security and was also being tested for contamination with radioactive polonium-210.

Italian security expert Mario Scaramella told The Associated Press he was in London to cooperate with the police investigation into the death of Alexander Litvinenko, a KGB agent turned Kremlin critic who died last week.

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Scaramella said he was being protected by a security team and would be tested for traces of polonium-210, the radioactive element found in Litvinenko's body. Scaramella, an academic who helped investigate KGB activity in Italy during the Cold War, declined to say whether he would be questioned by police.

Scaramella has said that he met Litvinenko at a London sushi restaurant on Nov. 1, the day the former spy reported falling ill. Scaramella said he showed Litvinenko e-mails from a confidential source identifying the possible killers of Russian investigative journalist Anna Politkovskaya and listing other potential targets for assassination — including himself and Litvinenko.

Litvinenko's unsolved death — blamed by the ex-spy and his Russian emigre allies on Moscow — has cast a shadow over British-Russian relations. Moscow is important to Britain as an energy supplier and member of the Group of 8 most industrialized nations, but many are critical of human rights abuses and unexplained deaths, including last month's unsolved murder of Politkovskaya.

Blair said police were determined to solve the mystery surrounding Litvinenko's death, and said he would speak to Russian President Vladimir Putin about the case "at any time that is appropriate."

"The police investigation will proceed, and I think people should know that there is no diplomatic or political barrier in the way of that investigation," Blair told reporters during a trip to Copenhagen, Denmark. "It is obviously a very, very serious matter indeed. We are determined to find out what happened and who is responsible."

London police say they are investigating the case as a "suspicious death" rather than murder, although they have devoted a large anti-terrorist force to the investigation.

High doses of polonium-210 — a rare radioactive element usually manufactured in specialized nuclear facilities — were found in Litvinenko's body. In a deathbed accusation, Litvinenko blamed Putin for his poisoning. Putin has strongly denied the charge.

In the wake of Litvinenko's death, more than 1,100 people have called a health hotline over concerns they may be at risk from polonium poisoning, which is deadly in tiny amounts if ingested or inhaled. Sixty-eight have been referred to health authorities, the Health Protection Agency said.

Eight exhibited symptoms that health officials thought should be examined at a special clinic as a precaution, the agency said. The tests should take about a week.

Opposition leaders have demanded the government explain how deadly polonium-210 came to be in Britain.

Russia's top nuclear official on Tuesday denied the polonium used to poison Litvinenko could have been stolen from a nuclear facility in Russia.

"Allegations that someone stole it during production are absolutely unfounded," Sergei Kiriyenko, director of the nuclear agency Rosatom, told a news conference. "The controls are very tough."

Kiriyenko said Russia exports 8 grams of polonium-210 monthly, all of it to the United States. He said there had been no exports to Britain in five years.

A coroner will perform an autopsy on Litvinenko on Friday, "subject to appropriate precautions," in a bid to pin down the cause and circumstances of death, said the local authority responsible, Camden Council.

Doctors had sought expert advice on whether Litvinenko's radioactive body posed a threat to the doctors and technicians performing the post-mortem.

A coroner's inquest will be opened Thursday and then adjourned until the police investigation is complete, the council said.

Detectives on Tuesday continued to retrace Litvinenko's steps on the day he claimed he had been poisoned.

Traces of radiation have been found at six sites, including the central London office of self-exiled Russian billionaire and Litvinenko mentor Boris Berezovsky.

In a statement, Berezovsky confirmed that radiation had been found in his office, and said he had "complete faith in the British authorities and the police."

Litvinenko's friend Andrei Nekrasov told The AP that Litvinenko frequently visited Berezovsky's office to use the telephone, computer or photocopier.

"Berezovsky's office was open to him informally," Nekrasov said. "His routine typically consisted of moving around, hopping on a bus, meeting people. He was trying to be active and needed."

Polonium-210 also was found in a building in the posh Mayfair neighborhood that houses Erinys UK Ltd., an international security and risk management company.

Police also have found traces of radiation at a bar in London's Millennium Hotel, a branch of Itsu Sushi restaurant near Piccadilly Circus, Litvinenko's house in North London and a section of the hospital where he was treated.

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