Police said they were treating the call - made from a phone box in Well Hall Road, Eltham, south-east London just yards from where Stephen Lawrence was murdered - with extreme caution. It is believed, however, that officers yesterday questioned a number of people suspected of membership of the group. Extra uniformed officers have also been drafted in to Brixton.
Deputy Assistant Commissioner Alan Fry, head of the Metropolitan Police's anti-terrorist unit, said the man had dialled 999 at 6.06am yesterday morning and told the operator: "This is Combat 18. We claim responsibility for yesterday's [sic] bomb."
He said that although the caller had wrongly implied that the bomb had exploded on Sunday rather than Saturday, his officers were investigating the call. "We can only reiterate that no motive has been ruled out. We are keeping an open mind," he said.
He added: "We cannot rule out that this was a hoax call. We are treating it with extreme caution."
Fifteen people remain in hospital as a result of injuries caused by the explosion in Brixton, south London, at 5.26pm on Saturday. Two men are in danger of losing their sight.
Police said yesterday that the bomb contained between 6-10lbs (4.5kg) of metal nails of various sizes, the largest up to 7ins (18cms) long. Officers have recovered the dark Head brand sports bag containing the bomb, which was taken by a passerby just moments before the bomb exploded outside an Iceland supermarket.
Police said they were also studying a number of other leads and have been scrutinising hours of CCTV footage taken from three fixed cameras near the bus stop in Electric Avenue. However it is understood their investigation has been hampered because shortly before 5pm on Saturday - the time at which the bag was first spotted at the stop before a group of youths, unaware of its contents, moved it to the supermarket - the view was at least partially obscured by a broken-down bus.
Police are also reviewing CCTV footage from Well Hall Road. A camera has been installed in the road to protect a memorial to Mr Lawrence, the black teenager murdered by racists in 1993.
If Combat 18 is responsible for the attack, it will mark a shift in the organisation's tactics. While it carried out a letter-bomb campaign in 1997, in which it targeted people including the former Olympic swimmer Sharron Davies who is married to a black athlete, it has never before launched an "open" bombing campaign.
"They are violent. They are capable of such behaviour but if it was them why did they not claim responsibility earlier?" said Steve Silver, co- editor of the anti-fascist magazine Searchlight.
"But they have not carried out this kind of bombing before which makes me feel it is unlikely to be them, unless they are trying something new."
Either way, if the aim of the caller was to heighten tension among the black community in Brixton, he may well be successful. Lee Jasper, secretary of the National Black Alliance, said he believed that the bombing may well be the work of racists. A public meeting has been called in Brixton by the National Black Alliance and the National Assembly Against Racism to discuss the attack and how to keep alive the recommendations made in the Macpherson report.Reuse content