By Tom Geoghegan
The death bed statement by Alexander Litvinenko blamed Russian President Vladimir Putin for the poison he believes took his life. But will we ever know with certainty who was responsible?
As the speculation about what poisoned Mr Litvinenko comes to an end with the announcement that radioactive substance polonium-210 was probably to blame, the question of who was responsible persists.
The former spy's two meetings in central London on 1 November, in Piccadilly and Mayfair, may hold the key to the identity of his killer.
Friends of the 43-year-old have blamed the Russian security service (FSB), as Mr Litvinenko accused it of many abuses, including the bombing of several apartment blocks in Russia in 1999, which killed 300 people.
The former KGB agent made enemies
Others had linked his sickness directly to another focus of his criticisms, former KGB agent Putin.
Any involvement has been dismissed by the Kremlin as "nonsense", a sentiment echoed by Russia's foreign intelligence service.
The matter is now in the hands of Scotland Yard, which is investigating the case as an "unexplained death".
Security analyst Glenmore Trenear-Harvey, who met Mr Litvinenko several times, said the media focus on the Kremlin was "lazy" and bore the hallmarks of a John Le Carre novel.
"We have to put this in a historical context," he said.
"Litvinenko's last job within the FSB was heading up the anti-corruption unit and he discovered a lot of corruption there and made a lot of enemies within the KGB."
When Yeltsin broke the KGB into different agencies such as the FSB and the SVR, the majority of its members stayed on but some went into the Duma and a third group went into legitimate business, he said.
But a "murky bunch" went into what was known as the Russian mafia.
"My own belief, and this is speculation, is that it's not inconceivable that Anna Politkovskaya in her search for murderers within the Russian bank system discovered the contract killings were these former KGB people.
"She was killed and if Litvinenko indeed was privy to her investigations then it could well be that they will emerge as his killers."
Although the sophisticated nature of the poison suggested it could have come from the state, there was no motive, he said.
"There was no benefit to Putin or Russian intelligence services to have a highly publicised operation like this."
The case has caused huge interest in the UK
And despite the continued claims linking Putin, diplomatic relationships between the UK and Russia were unlikely to be affected, he said.
Alex Pravda, an expert in Russia foreign policy and a member of international analysis organisation Chatham House, believes it is too early to say who was responsible.
"There's a lack of clarity in all this. It's a matter of speculation and I think we have to wait until there's better evidence," he said.
And the lack of coordination between Russian government and other agencies made it difficult to point the finger with any certainty, he said.
What has characterised the Litvinenko case from the start has been the way one explanation has been quickly replaced by another.
It was thallium. No, it was radioactive thallium. No, it was a cocktail of drugs. No, it was a mystery object. Now polonium-210 has been identifed.
Considering it took such a long time to find out what poisoned him, the matter of who was behind it may never be resolved.