With Britain embroiled in the Nato action in the Balkans, a political motive for the bombing, although thought to be unlikely, could not be ruled out.
Brixton is notorious for drug dealing, and "Yardie" gangs have been active in the area. But in the past, they have settled scores with guns rather than bombs.
The area has also suffered its share of racial violence and some residents placed the blame on ultra-right groups.
But speculation focused on animal rights activists, who may have planned it as a "spectacular" to mark World Laboratory Animal Week, which begins tomorrow.
Militant animal rights groups, including the Animal Liberation Front, the "Justice Department" and the Animal Rights Militia, have previously been linked to violent attacks. Several researchers have had death threats and Dr Colin Blakemore, an Oxford University scientist, received a "pin bomb".
But in the past they have opted for conventional incendiary devices not nail bombs. They usually claim responsibility for actions within days.
Robin Webb, a spokesman for the Animal Liberation Front, said the Iceland chain was not a "legitimate" target because, among supermarkets, it had the most "ethical" policy. Iceland was the first to ban GM ingredients from its own-brand foods.
In 1995, extremists sent two booby-trapped parcels to William Waldegrave, the then agriculture minister, to protest the live-animal export trade. Ethicon, an Edinburgh-based health-care company, which ran an animal testing laboratory, and the Alaska Fur Company in Glasgow, were also targeted.
Last year, animal rights extremists said they would step up their campaign of "economic sabotage". Death threats were sent to staff at Huntingdon Life Sciences, which was under investigation for cruelty to animals.Reuse content