By Paul Reynolds
World affairs correspondent, BBC News website
Now: Litvinenko in hospital in London...
The suspected poisoning of the exiled former Russian agent Alexander Litvinenko has raised suspicions that this might be the work of his old Russian security service colleagues.
Friends of Mr Litvinenko, who is being treated in a London hospital, claim that the Russian President Vladimir Putin, himself a former KGB officer, might have sanctioned an assassination.
Update 22 November: the mystery deepens. University College hospital now says that "thallium sulphate poisoning is an unlikely cause of his current condition.
"We cannot rule out the possibility that Mr Litvinenko's condition was caused
by a radioactive material - including radioactive thallium - although not all of
his signs and symptoms are consistent with radiation toxicity."
So the cause of his illness is unknown.
Some Russia watchers in Britain have cautioned against making any assumption that Mr Putin or any Russian organisation was involved.
"There is no direct evidence linking this to Mr Putin," said Alex Pravda of Chatham House and St Antony's College, Oxford.
"You have to remember that an important aspect of Russian life at the moment is a lack of co-ordination between government, corporate and other organisations. We have seen this over the arguments about Shell's operations in Sakhalin.
"You should not assume, therefore, that an order, if there was one, came from the top. In Russia, a lot of things are done independently. But there is an obsession with security in Russia these days, and that permeates political life and could have influenced people.
"And because the Western media, including the BBC, is making such play of this, the Russians will conclude that President Putin is being blamed, and this might play into his hands in that the West will be seen as hostile."
The finger of suspicion has been pointed at the Russian security service the FSB because Mr Litvinenko has emerged as one of its most severe critics, accusing it, for example, of once trying to assassinate the businessman Boris Berezovsky, now another leading exile in London.
And then: Litvinenko published a book of accusations against the FSB in 2002
He even accused the FSB of bombing blocks of flats in Moscow in 1999, killing more than 300 people. The motive in that case, he claimed, was to whip up support for a war against Chechnya.
Mr Berezovsky has visited him in hospital. Afterwards he told the Associated Press: "It's not complicated to say who fights against him. He's Putin's enemy. He started to criticise him and had lots of fears."
Another friend, Alex Goldfarb, who is also close to Mr Berezovsky, said: "Nobody's saying that Putin personally ordered it, though it's very likely."
And another former KGB exile in London, Oleg Gordievsky, claimed in an interview with The Times: "Of course it is state-sponsored. He was such an obvious enemy. Only the KGB is able to do this."
Mr Gordievsky thought that Mr Litvinenko could have been poisoned by a Russian who met him for a tea on the day this is thought to have happened, 1 November. This Russian, who has not been named, had been imprisoned in Moscow but was freed, Mr Gordievsky said. (update: it now seems there were two Russians at this meeting, but again neither has been named.)
On the same day, Mr Litvinenko had a sushi lunch with an Italian contact Mario Scaramella, who has been following the investigation into the murder of the Russian writer Anna Politkovskaya, in which Mr Litvinenko was interested.
Later that day, he fell, but poisoning was not suspected for another 10 days.
He was interviewed by the BBC Russian service on 11 November and said he had been approached by someone with documents naming a suspect in the Politkovskaya murder. This person was presumably Mario Scaramella. However he had not actually read the documents or seen the name. Litvinenko said in the interview he was "in a state of being poisoned" but left it to others to decide who was blame.
Update: Mr Scaramella has told the BBC that Russia mafia figures might have been behind the incident.
"We know very well who are the enemies of Litvinenko. The work we did for years was to underline the links among Russian mafia and some high-level corrupt officers in the Russian government.
"I can only imagine that the people who he worked against ... may be interested to attack him."
The Kremlin has dismissed allegations that it was involved as "sheer nonsense" and there has also been a denial by the FSB, a rare statement according to Moscow correspondents.
Russian sources have blamed rivalry among exiles or have suggested that it was set-up.
Gennady Gudkov, a member of parliament and a former FSB colonel, said: "I would advise Litvinenko to stay off the counterfeit vodka. Boris Abramovich [Berezovsky], although a talented director, won't manage to pull off this performance."
The event is reminiscent of the Cold War poisoning of the Bulgarian exile in London, Georgi Markov, who was murdered with an umbrella tipped with a ricin filled pellet.
Thallium itself has been used before, by Saddam Hussein's agents to take revenge on political enemies.
But there have been more recent episodes which suggest that poison is still a secret agent's tool.
The President of Ukraine, Viktor Yushchenko, fell ill during the presidential campaign in 2004 and his face was disfigured. Dioxin poisoning was diagnosed.
This latest event throws a potential shadow over British and Western relations with Russia.
Russia has been something of a disappointment to Western governments over recent years, though the feeling is probably mutual.
A recent article in the Economist suggested that Russia was heading in the wrong direction and used what it called "the f word" to describe this. It was "a word that captures the paranoia and self-confidence, lawlessness and authoritarianism, populism and intolerance, and economic and political nationalism that now characterise Mr Putin's administration. It is an over-used word, and a controversial one, especially in Russia. It is not there yet, but Russia sometimes seems to be heading towards fascism".
Most Western governments would not go that far. But the very fact that accusations in this case can even be levelled at the Russian state authorities shows how far trust has been damaged.