Polonium 210 sounds as if it comes from a science fiction novel. Although it occurs naturally in the environment and is present in substances such as tobacco, it is absolutely deadly to human beings in the most minute quantities — an amount that would fit on the head of a pin can kill. It is tasteless and odourless and once ingested or inhaled it cannot be detected except by a complicated urine test. If the poisoner gets the dose right, it can take up to 30 to 50 days to kill by which time the murderer can be long gone. But if he gets the dose wrong, then it might kill him as well as his victim. And its very rarity means it is difficult to obtain and thus limits the field of investigation to those who might have had access to it. It can only be made in a nuclear reactor (its principal use is as an initiator in atomic weapons) and because its half life (the speed at which it loses half its potency) is only 138 days, it needs to be ordered close to the time of its intended use.

So, given all this, where does the finger point? Just before he died, Litvinenko himself accused the FSB, the old KGB, given a new name after the collapse of the Soviet Union. From his bed in the cancer ward of Barnett hospital, where, spy-like, he was being treated under a different name three weeks before his death, he said his old service had poisoned him to avenge his defection. "The bastards got me," he told a friend. Old and new defectors rushed to agree with him. Oleg Gordievsky, the most senior KGB officer to defect to Britain said, "He was fighting against the evil forces of Russia, against the KGB and against the authorities. It is the first time in the history of Britain that a British citizen has been killed by a hostile security service on British soil." Certainly the FSB must figure as prime suspect. It would have had access to a nuclear reactor and the scientists skilled in using polonium 210. It has "previous form". When it was still the KGB, or the Cheka, or the OGPU or the NKVD — it is a service with many names and guises — it became notorious in the twenties and thirties for boldness and ruthlessness in eliminating "enemies of the state". It struck frequently at White Russian emigres in France. Operating out of the Soviet Embassy or special "safe houses", an assassination team would locate the target, drive up alongside him in the street, often in daylight, shoot him dead and vanish before the police could react. The assassination of Leon Trotsky in Mexico in 1940 demonstrated how far the Soviet intelligence could reach. When it was considered politically important that the death of a target should remain unsolved, "defenestration" was the method favoured. The victim plunged to his death from a high window, leaving open the possibility that he fell, jumped or was pushed — accident, suicide or murder.

Poisoning has the same ambivalence. Other techniques remain a mystery to this day. Russian defector Walter Kriviksky, who had been a Soviet military intelligence "illegal" in Western Europe until his defection to the United States in 1937, was found shot dead in his Washington hotel room in 1941. The door was locked from inside and three suicide notes found in the room. But Krivitsky had told friends that the KGB was after him and that if he were to be found dead, then he had been murdered. Why would the FSB want to assassinate Litvinenko in such a blatant manner? One answer would be in order to punish him for what his former colleagues saw as his treachery. They considered that by defecting to Britain he had betrayed them. And by doing it in such an unusual and terrible manner, they would send a message to anyone else in the service contemplating changing sides.

All this said, it may not have been the FSB at all. Litvinenko’s work with the FSB involved operations against Moscow’s underworld. His assassin might well have been a Russian crime boss. But where would he have obtained the Polonium 210? And even if turns out to have been the FSB, it is unlikely that President Putin had anything to do with it — he is not so foolish as to be involved in such a contentious operation. This is a murder story that is going to run and run.