Unraveling The Plot

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British authorities were pursuing a British-born Pakistani who entered England through a Channel port two weeks before the blasts despite being on a security watch list as a suspected al-Qaeda member. London police said there was nothing yet to link him to the plot, but a Pakistani official told TIME that two British investigators traveled to Islamabad last week to check on his contacts and whether he went to the frontier region, where Osama bin Laden and his deputy, Ayman al-Zawahiri, may be hiding. Working with Britain and Saudi Arabia, the U.S. froze bank accounts of the London-based Movement for Islamic Reform in Arabia, which is believed to be an al-Qaeda front.

The great fear among counterterrorism experts is that the London bombings may signal the beginning of a wave of attacks. French terrorism expert Roland Jacquard says French and Spanish investigators have been asked by their British counterparts to collaborate on finding "two or even three other teams" of suicide bombers that could be part of the Pakistani-led networks responsible for the London attacks. Jacquard says European investigators, on the basis of their experience in cracking the cell that carried out last year's terrorist strike in Madrid, believe the support network behind the London bombings may include as many as 30 members, "and they've been told it's possible that they're on their way to France or other European countries." Investigators have not ruled out the possibility that the conspiracy may reach into the U.S. LaRae Quy, an FBI spokeswoman in San Francisco, says the bureau is "concerned the cell may be in contact with individuals here in the U.S. We are concerned there may be copycats."

Finding them won't be easy. The London bombers were what law-enforcement officials call "cleanskins"--people with little or nothing on their records to raise suspicion. How investigators managed to unravel some of the main elements of the London plot last week is the story of advanced forensics and furious, old-fashioned legwork involving thousands of police, security and intelligence officials. It is from those clues that investigators are racing to assemble a picture of how al-Qaeda might have pulled off the London attacks and what its members may be planning next.

AT ABOUT 10:30 P.M. ON JULY 7, THE PARENTS of Hasib Hussain called police to report that their 18-year-old son was missing. He had told them he was going to London "with some mates," and they hadn't heard from him. A police liaison officer visited the Hussains at their home in Leeds, an industrial city of 715,000 to which many Pakistanis immigrated in the 1960s. She collected a photo and the names of his "mates," which matched names on items like credit cards and driver's licenses recovered at several of the crime scenes. The photo of Hussain was given to police who were poring over tapes of closed-circuit television (CCTV) footage taken at King's Cross on the day of the bombings. They soon had a match. One tape showed a man who resembled Hussain talking with three other men at around 8:20 a.m. On the tape, the men confer briefly, then go their separate ways. At 8:50, the first three bombs went off.