Phillip Knightley: Ignore the conspiracies. Spies never forgive a traitor

The FSB has what in police parlance is called 'previous form'

Alexander Litvinenko's death is unlikely to be solved for months. There are as many theories about who killed the former KGB officer as there are reporters working on the story. For my money, the circumstantial evidence points to the FSB, who took over the KGB's role and for whom Litvinenko once worked.

Few organisations have access to Polonium-210. It is made in nuclear reactors, and with a half-life of 138 days cannot be stored; it has to be made to order. It is an almost-perfect murder weapon, although in one sense the murderer was unlucky. If Litvinenko had not died in London where all the facilities existed to detect the Polonium-210, the cause might have remained a mystery. Litvinenko himself was in no doubt. "The bastards got me," he told a friend.

Old and new defectors rushed to agree, Oleg Gordievsky among them. President Vladimir Putin, himself a former KGB officer, has vigorously denied this charge, as has the FSB. But the trouble is spies as a breed find treachery hard to forgive. And the FSB has what in police parlance is called "previous form". When it was still the KGB, or the Cheka, or the OGPU or the NKVD, it became notorious for boldness and ruthlessness in eliminating "enemies of the state".

In the 1920s, 1930s and 1940s, its officers struck frequently at White Russian émigrés in France. Operating out of the Soviet embassy or "safe houses", an assassination team would find the target, drive 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 - an icepick in the head - demonstrated how far Soviet intelligence could reach.

When it was considered politically important that a death should remain unsolved, "defenestration" was the method favoured. The victim plunged to his death from a high window, leaving the possibility that he fell, jumped or was pushed - accident, suicide or murder. Poisoning has similar ambivalence.

In 1978, Georgi Markov, a Bulgarian defector, died after a Bulgarian security service officer fired a ricin-tipped dart into his leg from an umbrella gun as on Waterloo Bridge. The KGB provided the equipment.

Other techniques remain a mystery. Russian defector Walter Krivitsky, who had been a Soviet military intelligence "illegal" in western Europe until his defection to the US in 1937, was found shot dead in his Washington hotel room in 1941. The door was locked from inside and three suicide notes were found. But Krivitsky had told friends the KGB was after him and that if he were to be found dead, then he had been murdered.

But the Russian services are not alone. Frank Olson was a civilian biochemist working on biological warfare for the US Army. He also had links with the CIA which felt he was talking too freely. In November 1953 he plunged to death from the 13th floor of a hotel in New York. In 1975, a congressional inquiry was told the CIA had been experimenting with mind-bending drugs and, unknown to Olson, he was a guineapig. The CIA said he had jumped while on the drugs. Olson's son Eric is convinced his father was murdered to silence him.

In Britain, the MI5 officer David Shayler served seven weeks in jail for breaking the Official Secrets Act by criticising MI5 and MI6 operations. The government has pursued the former MI6 officer Richard Tomlinson with legal actions after he criticised his service and revealed the names of some officers.

The former FBI agent Robert Hanssen is serving life without parole in a "supermax" US prison for passing secrets to Moscow, including the names of CIA agents there. He is allowed no visits, no letters, no phone calls and no reading matter. Spooks don't like disloyalty. The Russians thought Litvinenko was disloyal.