Detectives are close to identifying the suspected killers of Alexander Litvinenko, the former Russian agent, but it looks increasingly likely that the alleged assassins will escape prosecution.
Scotland Yard officers are also investigating whether the former spy was first poisoned with polonium-210 several days earlier than previously reported. Investigators believe Mr Litvinenko may have been contaminated twice, with the second attack taking place at a central London hotel several days after the first "hit".
Two Russian businessmen, one a former KGB officer and the other a former Soviet soldier, are the focus of the investigation by anti-terrorist officers from the Metropolitan Police. Both strongly deny any wrongdoing.
It emerged yesterday that traces of polonium-210, were found at a restaurant that is understood to have been used by at least one of the suspects - Andrei Lugovoi.
But, despite detectives making what one insider described as "good progress" and being "confident about how things are going" senior officers believe there is little chance of a prosecution. "The odds of getting someone to face trial at the Old Bailey are somewhere between slim and none," said a senior police source.
The Russian authorities are known to have been obstructive when nine British detectives travelled to Moscow last month to investigate the murder. Scotland Yard has been told Russian suspects will not be extradited and that they cannot reinterview anyone who has already been questioned, including the two Russian businessmen who met Mr Litvinenko on the day he fell seriously ill. Officers may return to Russia to try to question new witnesses or suspects.
The police have been drawing up a detailed timeline of events up to when Mr Litvinenko became seriously ill on the night of 1 November. The former lieutenant colonel in the FSB, the security agency that replaced the KGB, died from radiation poisoning on 23 November. Before his death, he blamed Russia's President, Vladimir Putin, and his regime for the murder - an accusation that has been strongly denied by the Kremlin.
Investigators initially believed Mr Litvinenko was first poisoned at the Itsu sushi bar in Piccadilly, central London, when he met Mario Scaramella, an Italian espionage expert, for lunch on 1 November. The Japanese restaurant was the first place that traces of polonium were detected.
But detectives are understood to be investigating whether Mr Litvinenko was poisoned several days earlier. They are examining his movements and meetings, particularly with Russian contacts, in the previous days.
Toxicology results from Mr Litvinenko's post-mortem examination revealed two "spikes" of radiation poisoning, suggesting he received two separate doses. The second attack is almost certain to have taken place at the Pine Bar of the Millennium Hotel in Mayfair. Eight hotel staff have tested positive for polonium-210. So has a tea cup.
It was at this hotel that Mr Litvinenko meet two Russians - Mr Lugovoi, a former KGB colonel, and Dmitri Kovtun. A third Russian, Vyacheslav Sokolenko, is also reported to have been part of the group.
After the meeting Mr Litvinenko returned home in north London where he fell ill that night.
Tests found radiation at all three hotels where Mr Lugovoi stayed after flying to London on 16 October. Polonium-210 was also discovered on two aircraft on which Mr Lugovoi travelled.
In the latest development, traces of polonium have been discovered in the Pescatori restaurant in Dover Street, Mayfair, where Mr Lugovoi is understood to have dined before 1 November. Mr Lugovoi claims he is being framed.
The key figures in the investigation
* ANDREI LUGOVOI
The former KGB officer met Mr Litvinenko at a hotel on the day he fell ill, to discuss a business deal. Traces of the same type of radioactive element, polonium-210, which was used to killed Mr Litvinenko, have been found at three hotels where Mr Lugovoi stayed, at a London restaurant where he ate, and on two flights he took to Britain from Russia. This contamination appears to have occurred before Mr Litvinenko fell ill. Mr Lugovoi is now in Moscow where he is being treated at a clinic for mild radiation contamination. He denies any involvement in the murder.
* DMITRY KOVTUN
The Russian businessman and former Soviet soldier also met Mr Litvinenko the day he fell ill. He is being investigated by the German authorities for allegedly illegally handling polonium-210, which they believe was smuggled from Russia through Germany to Britain. This follows the discovery of radiation traces in Hamburg, where Mr Kovtun stayed the day before he travelled to meet Mr Litvinenko. He denies any involvement in the murder plot, and has been treated for radiation contamination in Moscow.
* VYACHESLAV SOKOLENKO
An associate of two men at the centre of the murder case, he has been questioned by British and Russian detectives. The private security officer and former Soviet soldier met Mr Litvinenko at the Millennium Hotel, along with two Russian contacts. Mr Sokolenko said he did not know Mr Litvinenko, and had gone to watch Arsenal play the Russian side CSKA Moscow.
* MARIO SCARAMELLA
The self-styled Italian "academic" and KGB specialist met Mr Litvinenko at a sushi bar on the day he fell ill. He said he met his friend to discuss a death threat. He is now in prison in Rome after being accused of deceiving Italian police about an assassination plot and impersonating a spy. Mr Scaramella is said to have wanted to use Mr Litvinenko, a former Russian agent, as a source for research into Italian politicians and allegations of links to the Russian security services. Mr Scaramella denies any wrongdoing. Tests at University College hospital, London, confirmed he had been exposed to Polonium-210.
* LEONID NEVZLIN
Friends of Mr Litvinenko fear that the Kremlin will use the investigation into the critic of President Putin to settle scores. Russian prosecutors have alleged that Mr Nevzlin, a former executive from the oil giant Yukos, could have ordered Mr Litvinenko's poisoning. Yukos has been dismembered and is bankrupt after facing billions of dollars of tax claims. Mr Nevzlin, who fled to Israel in 2003, has attacked President Putin for what he says was the politically-motivated destruction of Yukos.
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