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. law- enforcement 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 plotters' 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. Details started to emerge, and it became clear over the last couple weeks the nature of the threat and the individuals," said the official, who like others interviewed for this story spoke on condition of anonymity.
A law-enforcement bulletin issued Thursday by the Department of Homeland Security and the FBI described the conspiracy as "international in scope" but said there was no evidence that the plotters or any accomplices had set foot in the United States. "This plot appears to have been well planned and well advanced and in the final stages of preparation," the bulletin stated.
One U.S. intelligence source, however, 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 10 different 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. "There were different data sets about their intentions over time, that evolved over the period of time that we were following this. It did seem in more recent days to have centered upon carriers that had direct, nonstop flights between the UK and U.S.
"The real focus was to blow up airliners and the people on them," he added.
Counterterrorism officials said the basic outline of the conspiracy was known for several months. Investigators from New York to Islamabad 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 and assemble evidence for prosecutors.
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 locate or identify, U.S. officials said.
British Home Secretary John Reid said that "the police are confident that the main players have been accounted for" and were in custody. But U.S. and European authorities said the widespread ban against carrying liquids onto flights was imposed because investigators were worried more conspirators could be at large.
British police conducted numerous searches and raids Wednesday and Thursday as they detained at least 24 people in three cities.
British officials would not say if they were able to recover any physical evidence or bombmaking materials. Neither would they describe what kind of liquid explosives the suspects were allegedly planning to use.
U.S. counterterrorism officials said the explosive was a peroxide- based compound but would not give further details. One reason, they said, is that the chemicals are easily obtainable and are difficult to detect by airport security screeners.
Authorities are trying to "figure out how to prevent something similar from getting through," a senior U.S. intelligence official said. "We're learning what you can about this material, whether it opens another hole in our security, and we have to figure out how to close it."
