High levels of , who in his last days blamed President Vladimir Putin for his poisoning.
The radioactive isotope polonium-210 was found at the former Russian spy's North London home, reports CBS News correspondent Richard Roth, as well as at the fashionable Mayfair area's Millennium Hotel's dimly lit bar and the Itsu Sushi restaurant near Piccadilly Circus.
Litvinenko had meetings in both spots before he was taken to the emergency room suffering from stomach pains and nausea.
Officials called this poisoning case "unprecedented," Roth reports.
"For somebody to have this level of radiation they would either have to have eaten it, inhaled it or taken it in through a wound," Pat Troop of the British Health Protection Agency told reporters.
Polonium-210 has industrial use, and minute amounts occur naturally, but its radiation can't penetrate the skin; it's only deadly if it gets inside the body, Roth reports. Whomever killed the former spy, say poison experts, needed special knowledge and skill to deliver the fatal dose.
Although Litvinenko had been critical of Putin and his government, he was not widely known until he fell ill, CBS News Moscow Bureau Chief Beth Knobel reports.
On the morning of Nov. 1, he went to the hotel to see another ex-KGB spy, Andrei Lugovoy, who was in London to attend a soccer match involving the Russian team CSKA Moscow, and two other men Litvinenko had never met before.
Lugovoy said in Moscow on Friday that he was accompanied by a friend named Dmitry Kovtun and a third man he didn't identify. He told Russian media that Litvinenko discussed a business venture and said he was homesick for Russia.
Friend Alex Goldfarb said Litvinenko reported having a cup of tea during the meeting, but Lugovoy said he didn't recall the former agent either eating or drinking.
Litvinenko's friends said his patriotism coupled with a sense of false protection from his British asylum prompted him to reach out to potential Russian dissenters who might have bolstered allegations that Putin's government was involved in corruption in the spy service.
"Alex was open to approaches from people who said they had information about abuses in Russia," Goldfarb told The Associated Press. "He would meet people without precautions, because he felt as a British citizen he would be protected from falling prey to Russian government forces."
While at the hotel, Litvinenko told the group of his next meeting, Lugovoy said.
Litvinenko had arranged to see a contact who claimed to have information about the slaying of a friend, investigative journalist Anna Politkovskaya — a death that critics of the Russian government have blamed on state security forces.
That meeting was the afternoon of Nov. 1 at Itsu Sushi, a frequent rendezvous spot for Litvinenko and his friend Italian academic Mario Scaramella.
Scaramella said he brought along an e-mail purporting to list emigres to Britain being targeted by Russian agents as well as the identities of Politkovskaya's killers. The academic declined to say who sent the e-mail to him.
Litvinenko ate a bowl of soup before he headed for a table where the two could discuss the secret e-mail, Goldfarb said.
Akhmed Zakayev, a Chechen rebel exiled in London and a Putin critic, said he saw Litvinenko after the sushi meal and said the former spy was excited by the meeting. But Goldfarb said that after Litvinenko carefully analyzed the four-page e-mail, he doubted its authenticity.
Hours after the sushi bar meeting, Litvinenko was taken to Barnet General Hospital in north London with stomach pains. His condition gradually worsened over the next two weeks.
In a weakened state, he gave an interview to British Broadcasting Corp.'s Russian Service, saying for the first time that he thought he had been poisoned.
Unable to determine the cause of his illness, doctors transferred Litvinenko to a specialist unit at University College Hospital in central London on Nov. 17.
He died late Thursday, and investigators announced Friday that he had been poisoned with radioactive polonium-210, a rare substance.
"We will trace possible witnesses, examine Litvinenko's movements at relevant times, including when he first became ill and identify people he may have met," said Peter Clarke, head of London's anti-terrorist police.