Rush Hour Terror

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Initial responsibility was claimed by a little-known group that said it spoke for al-Qaeda, declaring the attacks retaliation for British support of the U.S.-led wars in Afghanistan and Iraq. Two days after the attacks, British police said no arrests in connection with the bombings had been made. But late last week a British official told TIME that the investigation is gravitating toward the possibility that as in Madrid, the attacks involved al-Qaeda-linked Moroccans, perhaps drawn from Britain's Moroccan community, coupled with outside guidance and bombmaking help. The official says authorities believe there may be links between the London bombers and those behind the Madrid attacks. "There's a lot of concern that the group is still here," the official says. "It may not presage any imminent attack. Maybe the greater danger is that they go dormant for weeks, if not months. It's a very considerable worry."

The links to the Madrid bombings are tantalizing. London's Sunday Times reported that Spanish security sources are said to have warned four months ago that Mustapha Setmariam Nasar, 47, a Syrian, had identified Britain as a likely target and had set up a sleeper cell of terrorists there. Eric Denécé, who heads the French Center of Intelligence Research in Paris, says that "there is some evidence" that Nasar helped plan the Madrid attack, and that "it wouldn't surprise me at all if he's found to have overseen London from afar as well."

According to a confidential report produced the day after the bombing by a private London security firm, Aegis Defense Services Ltd., which was seen and read by Pentagon officials, the team was probably four to six strong, although it is technically feasible that one or two bombers conducted the attacks. A British official says that based on the method of the attacks, "they would have needed quite a number of people, possibly as many as 10." The Aegis report says it is possible that the explosives were "constructed by an experienced bombmaker, possibly coming to the U.K. for that very purpose."

Investigators are also looking into whether Abu Mousab al-Zarqawi, al-Qaeda's top operative in Iraq, may have helped supply explosives for the London bombers. Meanwhile, a U.S. intelligence source tells TIME that last Friday a Pakistani was detained outside London at Stansted Airport, allegedly with a map of the Underground system and the three bombed train stations circled. A British official confirmed that a Pakistani had been arrested but said there was no known connection between the event at Stansted and the bombings. A source close to the interrogation of Abu-Faraj al-Libbi, a Libyan arrested in Pakistan who has been in U.S. custody for six weeks and is suspected of being Osama bin Laden's third in command, says al-Libbi told interrogators about the possibility of attacks in London and had in his possession city and Underground maps of London. U.S. authorities said there was no evidence of an imminent threat against the U.S., but a senior U.S. intelligence official told TIME that the FBI and other law-enforcement agencies have stepped up surveillance of possible terrorists or terrorist sympathizers on American soil.