As the Metropolitan Police insisted they were keeping "all avenues" of inquiry open, sources suggested that the police did not believe it was the work of an extreme right-wing group such as Combat 18. Gerry Gable, the publisher of the anti-fascist magazine Searchlight agreed. "This is a horrific act. If this turns out to have a racist basis we do not believe it will an organised group but a crazy individual," he said.
Mr Gable, whose organisation monitors racist activity in Britain, says that the extreme-right groups are in disarray at the moment in the wake of the inquiry into the murder of the black teenager Stephen Lawrence. "Perhaps it some lone extremist who has sat at home watching the news and seen the sympathy for the Lawrence family and has got angry," he said.
However, Brixton-based black rights campaigner Lee Jasper said the bomb could have been the work of a racially motivated far-right group. Brixton has a high proportion of ethnic minority residents and is well-known as a centre of black culture and the scene of race riots in the Eighties.
Mr Jasper said: "What we could potentially be witnessing is a backlash against the huge amount of coverage given to the Stephen Lawrence case. A nail bomb in the Brixton market could only be intended to inflict the maximum damage possible to the black community."
Mr Gable was himself was the target of letter-bomb attack by right-wingers nearly five years ago. The device was sent to his home but he became suspicious of the package and called in the bomb squad who defused it.
In 1995, Combat 18 - which gets its name from the initials of Adolf Hitler - were behind a plot to send parcel bombs to sports stars in mixed- race relationships. A key figure in the British terrorist group ordered Danish neo-Nazis to post bombs to figures, including swimmer Sharron Davies and boxers Frank Bruno and Kriss Akabusi. Three Danes were later jailed for the plot.
Neither would the Brixton bomb be the first time the extreme right have manufactured a nail bomb. The British National Party leader, Anthony Lecomber, was jailed for manufacturing a nail bomb. In 1985 he was on his way to plant the device outside the left-wing Workers Revolutionary Party headquarters in Clapham, south London, when it went off prematurely.
It certainly would not be hard for a single fanatic to make a nail bomb. Instructions for bomb-making have appeared in anarchist and survivalist manuals and on the Internet. The Nebraska-based national socialist organisation of Gary Lauck was suspected in the mid Nineties of mailing out a computer disc called "Endsieg" (Final Victory) to sympathisers which contained bomb-making instructions.
The Home Office minister Kate Hoey moved to play down the idea of a race motive in this weekend's attack. Ms Hoey, whose Vauxhall constituency is near the scene of the bombing, said: "I would not want to rule anything out but I would be very surprised if there is any kind of racist motive in it."
Scotland Yard has already ruled out the involvement of Northern Ireland paramilitary groups. Commander Hugh Orde, Metropolitan Police head of crime for south-west London, said he was otherwise looking at "all avenues". "Clearly the person who set this off is disturbed and has no regard for human life," he said.
Another possible theory is that the bomb was to be used in a dispute involving Yardie gangsters, who have been implicated in a recent series of shootings and murders in the area. The Jamaican gangs are notorious for gun crime and extreme violence, usually associated with the drugs trade. Although the feuds are becomingly increasingly violent, murders are most commonly shootings.
Some observers believe that the crude device was the work of a lone bomber with a grudge. In 1989, science student Matthew Williams was jailed for attempting to use a nail bomb to bring carnage to a Liverpool shopping street simply because he "hated people".
The Brixton bombing also occurred in the same week that the Mardi Gra blackmailer, Edward Pierce, who had set off explosive devices at supermarkets and shopping precincts across London, was jailed for 21 years.
But James Wyllie, an international security expert at the University of Aberdeen, said the bomber was almost certain to have had a political motive and the most likely explanation was that the attack was linked to the Nato action in the Balkans. He said: "The nature of Serbian resistance is such that sooner or later, there will be signs of Serbian discontent running right through Europe the same as we have seen with the Palestinians."Reuse content