Former KGB agent Andrei Lugovoi is the prime suspect in the poisoning of Russian exile Alexander Litvinenko in London last year.
Andrei Lugovoi has denied involvement in the murder
Mark Franchetti of BBC Two's This World has gained unprecedented access to him.
Andrei Lugovoi is a man with a lot on his mind.
Scotland Yard suspects him of killing Alexander Litvinenko - the former KGB officer poisoned in London with polonium-210 last year - in what is the world's first assassination known for sure to have been carried out with a radioactive substance.
As a result, he is also the man at the centre of the worst diplomatic crisis between Britain and Russia since the end of the Cold War.
Reason enough, one might think, to keep a low profile.
But since the Crown Prosecution Service announced five months ago that it wants Russia to extradite Lugovoi to put him on trial in Britain, the 41-year-old Russian businessman and former KGB bodyguard has done everything but keep out of the public eye, regularly giving interviews and press conferences.
So if you were under the impression that Lugovoi is a man on the run, think again.
He may not be able to travel outside the former Soviet Union for fear of being arrested and extradited to a London court but at home, far from being a fugitive, Britain's most wanted is a celebrity.
Next week Lugovoi is also to stand for the Russian parliament as a candidate for the LDPR, the ultra-nationalist party.
It may come as a surprise that a man accused of murder has a political future, but in Russia many die-hard patriots are prepared to vote for him.
It is partly to understand why, as well as try to shed some more light on how Lugovoi became embroiled in the Litvinenko case, that I spent so much time with him over the last few months.
Mark Franchetti accompanied Lugovoi on one of his camping trips
Obviously, I did not expect to crack the case but what interested me was who was Lugovoi the man, and what does his story say about modern Russia.
Citing Russia's constitution which bars citizens from being extradited to be put on trial abroad, the Kremlin has flatly refused to hand over Lugovoi.
Scotland Yard, meanwhile, is adamant that he poisoned Alexander Litvinenko by slipping a dose of polonium-210 in his tea during one of many meetings between the two men in London last year. It also suspects that he acted on orders of Russia's FSB, the former KGB.
Lugovoi, who was also contaminated with the deadly dangerous isotope, vehemently denies any involvement and claims to be a victim of a complex conspiracy involving British intelligence and opponents of Vladimir Putin, the Russian president.
"I was framed," said Lugovoi, who was once a KGB officer and is now a wealthy businessman.
Mark Franchetti has been a Moscow correspondent for 10 years
"This was a conspiracy to slur Russia and the Kremlin. I had no motive. I am a rich man, I am a successful businessman, I have a family. I would not do it for money or revenge. Whichever way you look at it there is no motive.
"Can one come to London and cold-bloodedly kill, and still behave calmly as I do - even though it hasn't been at all easy for me for the last year?" he adds.
Used or tricked
Several people I interviewed, including a former KGB general, believe that if Lugovoi did take part in Litvinenko's murder, he was used without knowing it.
According to this theory Lugovoi was recruited by Russia's security services but deceived by them.
It is possible, experts in covert operations say, that he was tricked into believing that the substance used against Litvinenko was a truth serum, or some debilitating drug.
"Let's, for the sake of argument, assume I had been in charge of such an operation," speculated Alexei Kondaurov, who spent 22 years in the KGB.
"And let's assume Lugovoi was involved. I would have told him as little as possible. Agents are used all the time without knowing the full details of an operation. If he had something to do it, he did not know it would lead to death."
Lugovoi went to an elite military college and joined the KGB aged 20
It is a theory I put to Lugovoi in our final interview. Self-assured as ever, he denied any involvement and claimed he was framed, not by Russian special services, but by MI6.
"MI6 wanted to kill him in order to discredit Russia's government," said Lugovoi, "because now in the West they all say that Litvinenko was killed on Kremlin and FSB orders."
In Russia the truth is often elusive, and the real story about who was behind Litvinenko's murder is unlikely ever to come out.
But as Lugovoi prepares to run for parliament, in Russia those prepared to vote for him could not care whether or not he is guilty.
Many think he is the innocent victim of a conspiracy to sully Russia. To those who think he did kill Litvinenko he is guilty of nothing more than defending his motherland.
Either way, he remains a free man and the Kremlin will never hand him over. For now at least, Britain's most wanted is a rising star.
This World: Britain's Most Wanted will be broadcast on Sunday 25 November 2007 at 1900 GMT on BBC Two.