It was a slow, agonising death. Whatever had poiosoned Alexander Litvinenko destroyed his bone marrow and liver, and eventually triggered a massive heart attack.
Not until shortly before the former Russian counter-intelligence official died last Thursday in a hospital in Britain, the country that had given him citizenship, did doctors finally discover why his life was ebbing away. He had been poisoned with polonium-210, a radioactive metal unknown to most medical experts. By the time his condition had been diagnosed, it was too late.
Polonium-210 is so dangerous that it may be impossible to carry out a conventional post-mortem on Mr Litvinenko. It is even possible that his remains will have to be disposed of in a manner that prevents any risk.
Since the death of the 43-year-old defector, radiation has been discovered at his London home, at a hotel in Grosvenor Square where he met two Russians, and at a sushi bar in Piccadilly, where he had lunch with an Italian academic who, it is claimed, declared that both of them were on a hit list. Those are almost the only undisputed facts in a sinister affair that has revived memories of the cold war and raised the spectre of terrorism.
President Vladimir Putin of Russia and his Federal Security Service, the FSB - the successor to the KGB in which Mr Litvinenko was a lieutenant-colonel - have been implicated in the accusations being flung between London and Moscow. So have insurgents in the savage war in Chechnya, their sympathisers in Britain, and the circle of exiles around the fugitive Russian billionaire, Boris Berezovsky, who, like Mr Litvinenko, was given political asylum in this country.
The poisoning on British soil of the former spy has not only set in motion an investigation on an unprecedented scale, it has aroused fears that the murky intersection of business and politics in Russia, which has seen a succession of unsolved murders, has been imported here.
Scotland Yard's anti-terrorist branch, along with forensic experts and nuclear scientists from Aldermaston Weapons Establishment, are working round the clock to establish exactly how and when Mr Litvinenko ingested the polonium. His death has also triggered a health scare. Officials yesterday urged anyone who came into contact with the defector, who was effectively radioactive, to call a special helpline number.
John Reid, the Home Secretary, was so concerned when radiation was detected that he convened an emergency meeting of cabinet ministers and senior officials of Friday morning. The official reaction - in which Pat Troop, head of the Health Protection Agency, has delivered public reassurances - followed closely protocols for a failed "dirty bomb", rehearsed by the Civil Contingencies Secretariat.
Exactly how one of President Putin's fiercest critics came to ingest polonium remains unanswered. Scotland Yard is not officially treating this as a murder inquiry but as an "unexplained death", and intelligence sources have told The Independent on Sunday that they have doubts over the former spy's version of how he first became ill.
But from the start, supporters of Mr Litvinenko, who came to this country six years ago and recently received British citizenship, have pointed the finger at associates of President Putin. This newspaper has learnt that Mr Litvinenko - known as "Sasha" to his friends - said he was on the verge of revealing the name of the assassin who gunned down Anna Politkovskaya, the Russian journalist murdered in Moscow in October, shortly before he was poisoned.
In an interview with the IoS, Lord Rea, the Labour peer and a friend of Mr Litvinenko, said he told members of London's Frontline club at a private gathering that he was "pretty sure" who killed Ms Politkovskaya, who exposed Russian human rights abuses in Chechnya.
"[Litvinenko] said he knew the person who did it," said Lord Rea, a supporter of the Chechen cause. "It's speculation, but it's possible that someone at that meeting heard that, which could explain the timing of the poisoning."
Detectives were told by the defector that he first became ill on 1 November, the day he had tea with another former KGB spy, Andrei Lugovoy, a one-time bodyguard who now runs a security firm. Also at the meeting in the Millennium Hotel in Grosvenor Square was another man initially identified by Mr Litvinenko as "Vladimir".
Speaking in Moscow, however, a Russian called Dmitry Kovtun told a national newspaper yesterday that he was the mysterious third man, and that he was "baffled" and "angry" to be linked to the poisoning.
The next appointment in Mr Litvinenko's diary was at Itsu sushi bar in London's Piccadilly with Mario Scaramella, an Italian academic who speaks fluent Russian and has good contacts in Moscow. It was here that the Italian disclosed that both he and Mr Litvinenko were on a hit list.
Scotland Yard has already confirmed that traces of polonium have been discovered at the Millennium hotel, on tables and dishes at the sushi restaurant and at the former intelligence agent's home in Muswell Hill.
All the potential suspects in the case - the three people he met on the day the Russian believed he was poisoned - have co-operated with police. All have denied they have had any involvement.
Akhmed Zakayev, a Chechen dissident and friend of Mr Litvinenko, said he was confident that the police would get to the bottom of how his friend was poisoned. Speaking through a translator, he said: "Whoever did this does not want a witness to their crimes, the same people who did not want Anna Politkovskaya to be a witness. This is not about someone being Russian or Chechen. It is about the safety of British citizens."
So how did the former intelligence agent come to ingest the lethal dose of polonium? Could he have drunk it, or was it sprinkled on his food? And how did the perpetrator of the crime come to be in possession of such a rare substance? Once it is absorbed by the body, the metal is said to be several million times more deadly than cyanide.
Professor John Henry, the toxicologist hired by Mr Litvinenko's family, said that polonium was the "perfect assassin's tool". "It would take a pinch the size of a speck of dust, so it would need to be either dissolved in a liquid or a spray," he said. "This must have been a massive amount for traces to have been left behind after so long."
Experts agree that this was no crime carried out by amateurs. Polonium cannot be obtained by surfing the Web, and has to be used within a limited time, before it loses its impact. Large-scale production such as in a nuclear reactor would be needed to produce sufficient amounts to cause death. Another way of obtaining it is from depleted uranium shells . It is also used in the photographic industry as a static eliminator.
Staff at the hospital where Mr Litvinenko was treated are understood to have used a Geiger counter to determine if he was the victim of radiation but initial tests showed up negative, because polonium cannot be easily detected externally once it has been absorbed.
Dr Andrea Sella, a lecturer in chemistry at University College London, said: "You can't make this at home. This is in a different league. This is not some random killing ... These people had some serious resources behind them."
When news first began circulating that a former Russian security officer had been poisoned in London, the Kremlin was swift to attempt to discredit the story, with officials questioning why it had taken Mr Litvinenko so long to admit himself to hospital. The FSB, Russian legislators and political analysts insisted that the Russian government did not have a credible motive for murdering him.
"Mr Litvinenko was not the kind of person, for whom [it would make sense] to smear bilateral relations," a spokesman for the Russian Foreign Intelligence Service, Sergei Ivanov, told Trud newspaper. It "was absolutely not in our interest to do this."
Mr Litvinenko's name had faded in Russia since 1998, when as a young FSB officer with the organised crime unit, he accused his own agency of asking him to assassinate Mr Berezovsky. He spent nine months in prison awaiting trial on charges of abusing his office, but was acquitted and fled to Britain in 2000, where he claimed political asylum.
Gennady Gudkov, a former FSB officer, said killing Mr Litvinenko made no sense for the Russian state. Higher-ranking KGB defectors, such as Oleg Kalugin, have knowledge of potentially deadlier secrets, but "are alive and well" abroad, said Mr Gudkov, who is a member of the State Duma's Security Committee.
Mr Kalugin, chief of the KGB's foreign counterintelligence department from 1973 to 1980, has been living in the US since the mid-1990s. He has openly criticised his former KGB colleagues and testified against US Army Reserve Colonel George Trofimoff. Based on Mr Kalugin's testimony, the officer was convicted of spying for the Soviet Union and sentenced to life in prison in 2001.
Mr Gudkov said that the key to Mr Litvinenko's murder lay in "an inside squabble" in Mr Berezovsky's inner circle. Mr Gudkov stressed that this was his personal opinion, adding that business interests or Mr Berezovsky's desire to smear the Russian state were involved.
The theories about Mr Litvinenko's murder are potentially damaging to both President Putin and Mr Berezovsky, said Valery Khomyakov, general director of the Council on National Strategy, a Moscow-based think-tank.
Mr Khomyakov dismissed theories that Mr Litvinenko was poisoned because he possessed information about the murder of Ms Politkovskaya. He and several other political analysts in Moscow believe the most plausible view is that Mr Litvinenko's former colleagues were avenging his defection. "Many suffered when Mr Litvinenko went public about the alleged plot against Mr Berezovsky," he said.
In a book financed by Mr Berezovsky, Mr Litvinenko accused the FSB of being behind the 1999 Moscow apartment bombings that killed more than 300 people. The atrocity was blamed on Chechens and used to justify Russia's invasion of Chechnya the same year. "This could have been a way to get back at Litvinenko and another former colleague, Putin," Mr Khomyakov said. Mr Putin spent five years spying for the KGB in Germany at the end of the Cold War.
Additional reporting by Maria Levitov in Moscow, Francis Elliott and Raymond Whitaker
Polonium: Fatal dose must have come from nuclear lab
Discovered by Marie Curie in 1898, and named after Poland, her native land, polonium has a silvery appearance and is soluble in liquids.
Polonium-210, the most readily available variant, has a half-life of 138.39 days. Other isotopes of the element can decay in milliseconds. Although it is extremely toxic and highly radioactive - just one milligram would emit as much radiation as five grams of radium - the metal emits short-range alpha rays, which would not be picked up by conventional radiation scanners. It was detected only in Alexander Litvinenko's urine.
Polonium-210 is found naturally in the human body, as well as in tobacco and uranium ore, but in minuscule quantities. Although a tiny speck can be fatal, the amount needed to kill would have to be made in a nuclear laboratory. The metal has to be ingested by breathing, eating or drinking, or through an open wound; it cannot be absorbed by skin contact.
Inside the body, radioactive waves pound cells, destroying them outright or causing genetic mutations. As it decays, polonium-210 generates great heat: half a gram creates 140 watts of energy. The metal was used by the Soviet space programme in the 1970s as a portable heat generator for Lunokhod lunar rovers.
The Theories: One terrible death... but many suspects in the frame
The history of Alexander Litvinenko, a former officer in the KGB and its successor service, the FSB, his accusations against the Russian government and his friendship with the exiled oligarch, Boris Berezovsky, have led to competing theories on who was responsible for his ghastly death.
MOSCOW DID IT: The defector's associates have accused President Vladimir Putin of being behind the killing. Russia sought to treat the allegation with disdain, but as the controversy grew, Mr Putin was forced to issue a formal denial. The official position is that the death is a tragedy, and should be left to the British police. Most believe the theory is implausible: if the Kremlin wanted to kill Litvinenko, why attract world-wide publicity? A quiet bullet would have been equally effective.
ROGUE FSB ELEMENTS: This is in many ways more damaging to Mr Putin. It has been suggested in Britain and in Russia that the President does not fully control the security service, and that he is not aware of everything it does. Litvinenko earned the enmity of former colleagues, first by saying he had been ordered to assassinate Mr Berezovsky, later by accusing them of a plot to justify the invasion of Chechnya. He also claimed that there was official involvement in the recent killing of the investigative journalist, Anna Politkovskaya. The FSB might have chosen to make his death as lurid as possible, to warnother potential whistle-blowers.
BEREZOVSKY IS RESPONSIBLE: This is the favourite theory of the Kremlin's supporters in Moscow, where the fugitive oligarch is vilified as a criminal plotter against Russian interests from his exile in Britain. Why the billionaire would want to dispose of an ally who had lent powerful support to his campaign against Russian abuses in Chechnya is less clear. There have been half-hearted claims that he or his circle might have wanted to create a "martyr" for the cause.
LITVINENKO POISONED HIMSELF: Another suggestion being heard mainly in Moscow. Why anyone would choose to kill himself in such a dreadful fashion is a mystery, and for what reason? Martyrdom? However, the British authorities have allowed the theory to gain some momentum by refusing publicly to rule out the possibility, and by hinting that Litvinenko's own account of the poisoning is flawed.
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