In the aftermath of the July 7, 2005, transit bombings in London, British authorities received a call from a worried member of the Muslim community, reporting general suspicions about an acquaintance.
From that vague but vital piece of information, according to a senior European intelligence official, British authorities opened the investigation into what they said turned out to be a well-coordinated and long-planned plot to bomb multiple trans-Atlantic flights heading toward the United States–an assault designed to rival the scope and lethality of the Sept. 11, 2001, hijackings.
By late 2005, the probe had expanded to involve several hundred investigators on three continents who kept dozens of suspects under close surveillance for months, even as some of the plotters traveled between Britain and Pakistan to raise money, find recruits and refine their scheme, according to interviews with U.S. and European counterterrorism officials.
Precise details of the plot–how many planes, their destinations and the date–remain unknown. The shape of the operation changed regularly as the would-be bombers considered which trans-Atlantic flights to target and prepared for a practice run, which was expected to take place in the next few days, U.S. officials said.
Investigators eventually pieced together enough information from a blizzard of stakeouts, tips and wiretaps to make clear that something big was in the works and that the preparations were nearing an end.
“It’s not like three weeks ago all of a sudden MI5 knew about this plot and went to work,” a U.S. law-enforcement official said, speaking of the British security service. “They’d had a concern about these guys for some time, for months.”
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One U.S. intelligence source said some of the British suspects arrested had made calls to the United States.
Investigators believe the London operation was composed of three distinct cells, whose members may not have been aware of one another or the extent of their assignment, U.S. officials said.
British officials suspect that as many as 50 participants and accomplices were involved, U.S. law-enforcement officials said. Internet searches made by the suspects suggested that they had considered targeting up to 12 flights, investigators said, although there was no evidence that those arrested had bought tickets or made reservations.
“It’s fair to say they were aiming for multiple flights, and some of the exact data of who they would deploy and how many might be in one deployment are somewhat ambiguous,” said Michael Jackson, deputy secretary at the Department for Homeland Security.
Counterterrorism officials said the basic outline of the conspiracy was known for several months. Investigators from New York to Islamabad, Pakistan, said they were briefed by their British counterparts late last year.
British and U.S. law-enforcement authorities decided against breaking up the cells right away in hopes that they could learn more about the origins of the network.
Some U.S. counterterrorism officials said plans originally were to allow the conspiracy to develop even further. But U.S. and British investigators suddenly decided this week to close down the operation after becoming increasingly worried that there were other bombers they had failed to find or identify, U.S. officials said.