Traces of the same rare, highly radioactive material that killed a former KGB agent were found Friday in the sushi restaurant and hotel he had visited, and in his home, setting off a frantic investigation to see if anyone else may have been contaminated by the "tiny nuclear bomb."

The rare radioactive element Polonium-210 was detected in Alexander Litvinenko's urine after he died Thursday in a London hospital. The Itsu sushi restaurant in London's Piccadilly and parts of the Millennium Hotel in Grosvenor Square were closed as investigators scoured for evidence in a death that rocked the government leaders in London and Moscow.

Litvinenko, in a statement written before his death and read Friday, claimed Russian President Vladimir Putin of being behind his murder, a charge Putin denied.

Britain's Health Protection Agency, meanwhile, said the finding of polonium-210 in the dead former spy's body was "an unprecedented event" and Britain's government convened a crisis committee in response.

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 exile Leonid Nevzelin said in Israel that Litvinenko's death could be linked to investigations into charges laid against ex-shareholders and former owners of the Yukos oil company.

Chemical experts, meanwhile, told the Times of London that a fatal dose of polonium-210 could only be produced artificially, by a particle accelerator or nuclear reactor.

"This is not some random killing. This is not a tool chosen by a group of amateurs. These people had some serious resources behind them," Dr Andrea Sella, a chemistry professor at University College London, said.

"Only a very, very small amount of polonium would need to be ingested to be fatal, but that depends on how pure the polonium is," said Dr. Mike Keir, a radiation protection adviser at the Royal Victoria Infirmary.

Keir said polonium poisoning was extremely difficult to detect because the type of particles it emits — alpha particles — do not penetrate outer layers of the body. It would also not set off airport radiation detectors, experts said.

Scientists claimed small amounts of polonium-210 — but not enough to kill someone — were used legitimately in Britain for industrial purposes and easily available.

To be used to kill, however, "much larger amounts are required and this would have to be manmade ... from a particle accelerator or a nuclear reactor," said Medical Research Council expert Dudley Goodhead.

Investigators theorized Friday night how the radioactive substance could have been ingested by Litvinenko, with Health Protection Agency officials offering a theory that it could have been sprayed on him and inhaled.

Litvinenko, meanwhile, literally spoke from the grave as a friend, Alex Goldfarb, read a statement the former Moscow agent wrote just before his death, in which he accuses Putin of poisoning him.

"You have shown yourself to have no respect for life, liberty or any civilized value. You have shown yourself to be unworthy of your office, to be unworthy of the trust of civilized men and women," Goldfarb read.

• Click here to read Litvinkenko's full statement.

Litvinenko, a fierce critic of the Russian government, had suffered heart failure and was heavily sedated as medical stuff struggled to determine what had made the 43-year-old critically ill.

"You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life," Litvinenko said in his statement.

Goldfarb said Litvinenko had dictated the statement before he lost consciousness on Tuesday, and signed it in the presence of his wife Marina.

Putin brushed off accusations Friday that he was responsible, saying that the Litvinenko's death was a tragedy but that he didn't see proof that it was a "violent death."

"It's extremely regrettable that such a tragic event as death is being used for political provocations," he said. "I think our British colleagues realize the measure of their responsibility for security of citizens living on their territory, including Russian citizens, no matter what their political views are. I hope that they won't help fanning political scandals which have no grounds."

Litvinenko's father, Walter, who had flown to London from Russia to be with his dying son, accused Putin of waging a polished public relations campaign, telling reporters that his son was killed on the orders of the Russian government "by a little, tiny nuclear bomb, so small that you couldn't see it". He went on to warn that the "people who killed him build big nuclear bombs and missiles and ... should not be trusted," the Guardian newspaper reported.

British health officials, meanwhile, scoured the neighborhood where Litvinenko is believed to have ingested the radioactive poison.

Health Protection Agency chief executive, Pat Troop, said that the high level found in the dead spy's urine indicated "he would either have to eaten it, inhaled it or taken it in through a wound."

"We know he had a major dose," she said.

"I've been in radiation sciences for 30-odd years and I'm not aware of any such incident," said Roger Cox, director of the agency's center for radiation, chemicals and environmental hazards.

Britain's home secretary John Reid, the country's top law-and-order official, said Litvinenkco's death was linked to a radioactive substance in his body.

He said experts had been called in to search for "residual radioactive material" at a number of locations as police investigate the cause of Litvinenko's death, and it was "linked to the presence of a radioactive substance in his body."

"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.

Earlier, London's Metropolitan police said in a statement that the case "is being investigated as an unexplained death."

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The former spy said he believed he had been poisoned on Nov. 1, while investigating the slaying of another Kremlin detractor — investigative journalist Anna Politkovskaya. His hair fell out, his throat became swollen and his immune and nervous systems were severely damaged, he said.

Just hours before he lost consciousness, Litvinenko said in an interview with The Times of London newspaper that he had been silenced.

"I want to survive, just to show them," he said in the interview was published in Friday's edition of the paper, copies of which were available late Thursday. They "got me, but they won't get everybody."

Doctors at London's University College Hospital said tests had virtually ruled out poisoning by thallium and radiation — toxins once considered possible culprits behind the poisoning.

"The medical team at the hospital did everything possible to save his life," hospital spokesman Jim Down said, confirming the Russian's death Thursday night.

"Every avenue was explored to establish the cause of his condition, and the matter is now an ongoing investigation being dealt with by detectives," he said.

Dr. Geoff Bellingan, the hospital's director of critical care, said extensive tests had failed to uncover what had caused Litvinenko to fall ill.

Earlier in the day, hospital officials said Litvinenko was deteriorating rapidly and family members and friends rushed to his bedside.

Goldfarb joined Litvinenko's wife Marina, his son Anatoli and father at the hospital.

"He went into a cardiac failure overnight and the hospital put him on artificial heart support," Goldfarb said. "He's on the ventilator, he's getting artificial resuscitation."

Anti-terrorist police were investigating the poisoning, which friends and dissidents allege was carried out at the behest of the Russian government. Litvinenko sought asylum in Britain in 2000, and has been a relentless critic of the Kremlin and the Russian security services ever since.

Litvinenko worked both for the KGB and for a successor, the Federal Security Service. In 1998, he publicly accused his superiors of ordering him to kill Russian tycoon Boris Berezovsky — now exiled in Britain — and a year later spent nine months in jail on charges of abuse of office, for which he was later acquitted, and which prompted his move to London.

On the day he first felt ill, Litvinenko said he had two meetings. In the morning, he met with an unidentified Russian and with Andrei Lugovoy, a former KGB colleague and bodyguard to one-time Russian Prime Minister Yegor Gaidar at a London hotel. Later, he dined with Italian security expert Mario Scaramella to discuss the October murder of Politkovskaya.

Scaramella told reporters in Rome on Tuesday that he had traveled to meet Litvinenko to discuss an e-mail he received from a source naming the killers of Politkovskaya, who was gunned down Oct. 7 at her Moscow apartment building, and outlining that he and Litvinenko were on a hit list.

Goldfarb said that he had a photocopy of the four-page e-mail and confirmed that it did read like the hit list described by Scaramella.

"What's in there confirms what Scaramella said. It lists several targets for assassination, among them are Politkovskaya, Litvinenko, Scaramella, Berezovsky and others," he said. But he refused to say who compiled the document, saying that it could jeopardize the police investigation into the poisoning.

Litvinenko refused to implicate any of the people he met on the day he said he believed he was poisoned.

Sky News, The Times of London and the Associated Press contributed to this report.

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