It's a murder mystery filled with intrigue reminiscent of the Cold War - there's a retired Russian spy poisoned by a radioactive substance, a slain investigative journalist and a shadowy fugitive billionaire.
It is the story of the agonizing death of Alexander Litvinenko, and the final chapter of this spy thriller has not yet been written.
The most crucial questions remain unanswered: Was Litvinenko's death murder? Who killed him? Where did they get the poison?
Most intriguingly, who might have ordered his death?
The tale began after Litvinenko, a former Russian intelligence officer, met with Mario Scaramella, an Italian security expert, in a London sushi bar Nov. 1. Scaramella passed Litvinenko a secret file purportedly showing that both men were on a hit list of Kremlin opponents.
Both somehow ingested polonium-210, a substance normally produced in nuclear reactors.
Litvinenko fell ill and died, blaming Russian President Vladimir Putin. Scaramella was exposed to a smaller amount and showed no signs of illness, doctors said Saturday.
Investigators have found traces of radiation at a dozen sites across London, including on three British Airways jetliners that were cleared to resume service Saturday. Litvinenko's wife was also contaminated with trace amounts of the poison, a friend said Friday.
Litvinenko told a reporter in June that a new Russian law would permit authorities to target its opponents abroad. He feared he was among them.
Another former Russian intelligence officer, Mikhail Trepashkin, wrote in a letter delivered Friday by human rights activists in Moscow that the Federal Security Service, or FSB, the main successor agency to the Soviet KGB, had created a hit squad to kill Litvinenko and other Kremlin foes.
Trepashkin, who is serving a four-year sentence in a prison in Yekaterinburg for divulging state secrets, said he warned Litvinenko of the threat during a meeting in August 2002.
The Kremlin has dismissed the accusations as fantasy.
The Guardian newspaper Friday reported that British intelligence sources suspect Litvinenko was the victim of a plot by "rogue elements" in the Russian state. Investigators suspect that several Russian agents may have entered Britain with a crowd of Moscow soccer fans shortly before Litvinenko met Scaramella, the newspaper reported.
Litvinenko's friends have little doubt that Russian authorities were somehow involved.
"These latest developments only reinforce our thinking that it was the Russian government or some element of (Russia's) political landscape that was behind this," said Alex Goldfarb, Litvinenko's friend and spokesman.
Goldfarb and others suspect he was targeted because he was investigating the death of journalist Anna Politkovskaya, a Kremlin critic shot to death in her apartment building in October.
Not everyone suspects the Kremlin in Litvinenko's death. Some of Putin's allies say the poisoning may be the result of a falling out among the government's enemies. This theory says the killer hoped to frame the Kremlin.
No one is naming any names, but Putin's supporters like to point out that Litvinenko's friend and sponsor was the fugitive Russian tycoon, Boris Berezovsky.
Berezovsky, a onetime Kremlin insider, is now a critic of the Russian president. Russia is seeking to extradite Berezovsky on fraud charges, but he was granted asylum in Britain in 2003.
"Litvinenko was moving in that community," said Bob Ayers, a security expert at the London think tank Chatham House. "It seems to me to be a very dramatic way of achieving their aim of discrediting Putin's Russia."
Litvinenko broke with the FSB in 1998 when he announced publicly that he had refused to obey an order from his superiors to kill Berezovsky.
Another theory holds that associates of Berezovsky's may have killed Litvinenko as part of some murky business dispute.
And some in Russia suggest Litvinenko may have been trying to deliver polonium-210 to Chechen rebels so they could build a conventional bomb that would spread deadly radiation.
They said Litvinenko met several times with Akhmed Zakayev, a representative of the late Chechen President Aslan Maskhadov. Zakayev also lives in exile in London.
No mystery is complete without a plot twist. In this tale, that is the case of Yegor Gaidar, Russia's former minister of economic development and also a critic of Putin. Gaidar fell ill on a recent trip to Ireland, and his doctors in Russia suspect he was poisoned. He is recovering in a Moscow hospital.
Irish doctors investigating his illness have concluded he was not poisoned by a radioactive substance, said a health official speaking to the Associated Press on condition of anonymity in line with government policy.
One of Gaidar's former bodyguards is Andrei Lugovoi, yet another retired FSB officer.
Lugovoi also met Litvinenko in London on Nov. 1 to discuss business. But Lugovoi suggested in an interview with the Russian newspaper Kommersant published Saturday that Litvinenko could have been poisoned weeks earlier than investigators believe.
Lugovoi said that he, a business partner and Litvinenko met in London on Oct. 17 in the office of Erinys UK Ltd., an international security and risk management company. Police have said that traces of radiation were found in the building that houses Erinys.