LONDON, July 8 - Investigators have concluded that the bombs that ripped through three subway trains and a bus on Thursday were relatively crude devices containing less than 10 pounds of explosives each. That finding supports a theory gaining momentum among the authorities that the plot was carried out by a sleeper cell of homegrown extremists rather than highly trained terrorists exported to Britain.
Crucial facts remained unknown in the case Friday evening. But senior British and American investigators said the morning rush-hour attacks in central London appeared to have been a low-tech operation, less expertly conceived than the cellphone-detonated train bombings in Madrid on March 11, 2004, which killed 191 people and wounded more than 1,800.
Officials said at least 49 people had been killed and 700 wounded in the London bombings.
But they said the death toll might have been much higher had the attack been carried out by experts - given the large numbers of commuters crowded into the transit system.
A senior British investigator said Friday that the authorities initially feared “hundreds of people” might have died, and he said they were both surprised and relieved by the relatively low death toll. There may be other signs that an inexperienced sleeper cell was at work, investigators said. The bombings have so far produced very few leads or possible suspects among the known extremist groups and cells in Britain and the rest of Europe. That could mean, the investigators said, that the bombers never crossed paths with the authorities and might not have been associated with known extremist groups in the past.
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