Russian detectives will head for London this week to launch Russia's own probe into the ever-expanding mystery of who poisoned ex-KGB agent Alexander Litvinenko and at least two others with radioactive polonium-210.
Russia's decision to mount a parallel investigation comes amid a spate of fresh discoveries of polonium traces around London, where Litvinenko was allegedly poisoned on November 1, and in a Hamburg apartment building used by one of his contacts on that fateful day.
Dmitri Kovtun, who owned the Hamburg flat, is currently "in critical condition" in a Moscow hospital with apparent polonium poisoning.
Another Russian who met with Litvinenko in London's Millenium Hotel that day, ex-KGB agent Andrei Lugovoi, has also apparently been taken ill with radiation sickness.
"Disruption in the functioning of some (of Lugovoi's) organs affected by radiation nuclides has been found," the independent Interfax news agency quoted hospital sources as saying. "Lugovoi's condition is considerably better than that of Kovtun, but he also has symptoms of contamination."
Experts say the picture is growing more confused by the day, and the expanding trail of polonium presents a baffling blizzard of bizarre clues.
"If properly transported, polonium shouldn't leave any traces at all," says Alexei Yablokov, a leading Russian scientist. "This whole story is growing more phantasmagoric with each new development. It's almost as if someone were deliberately working at it, to create all this frenzy."
The independent Russian investigation into Litvinenko's death would mean that any suspect found in the case could be put on trial in Russia, a spokesman for prosecutor-general Yury Chaika said. Moscow has already made clear that no extradition to Britain will be permitted.
Nine British detectives have been in Russia for the past week, following the trail of radiation that leads through at least two British Airways aircraft that flew between London and Moscow before Litvinenko's November 1 murder.
The British detectives spoke with Kovtun last Tuesday, before he fell seriously ill, but have yet to hold an interview with Lugovoi.
In her first public interview since last week's burial of her husband, Marina Litvinenko restated the widespread belief among Russian exiles that the Kremlin might have been behind the murder.
"Obviously it was not Putin himself, of course not," Mrs Litvinenko told journalists in London. "But what Putin does around him in Russia makes it possible to kill a British person on British soil. I believe that it could have been the Russian authorities."
Many Russian experts say they believe the murderers will be found among the back-biting Russian émigré community that Litvinenko inhabited, where intrigues and conspiracies allegedly abound.
In particular, they point to exiled anti-Kremlin tycoon Boris Berezovsky, Litvinenko's former employer who is wanted in Russia on a variety of charges, including planning to overthrow the state.
"The version about Berezovsky's involvement finds further confirmation," former chief of the FSB security service Nikolai Kovalyov told the official RIA-Novosti news agency last week. "The ultimate goal of the operation could have been further building-up of KGB-phobia (in the West), to claim that Russia is ruled by members of the secret services."
Russia, the world's biggest producer of polonium-210, makes about 8 grams of the substance each month, all of which is exported to the US for use in the printing industry.
Russia's top regulatory agency for nuclear materials has flatly denied that the polonium allegedly used in the Litvinenko killing could have come from Russia.
Six Russian facilities hold licences to work with the deadly alpha-emitting substance, but all their output is accounted for, according to Konstantin Pulikovsky, head of the Federal Oversight Service.
"When the subject began to be discussed in the press, we carried out additional checks everywhere," he said. "I can say with complete certainty that no deviations from the rules on storage and transportation of nuclear materials, including polonium, have been discovered at any structures of our fuel and nuclear complex".
British police visiting Russia will not be allowed to question another potential witness in the case, Russian authorities have said. Former security officer Mikhail Trepashkin, who was jailed in 2004 for "revealing state secrets", has said through his lawyer that he has key evidence concerning the murder of his ex-colleague Litvinenko.
But the Russian prison service has ruled out Trepashkin's bid to talk with the British, on grounds that a prisoner being held for the crime of treason is not permitted any contact with foreigners.
Meanwhile, former Russian acting Prime Minister Yegor Gaidar has insisted he was "definitely" poisoned during a trip to Ireland last month.
In an article in the Daily Vedemosti, Gaidar said his case was probably part of a pattern of provocations by enemies of Russia aimed at destabilizing the Kremlin.
"More likely than not, someone among the open or hidden enemies of the Russian authorities, those who are interested in a further radical deterioration of ties between Russia and the West, stands behind this," he wrote.