Wednesday night was a long and troubling one for Homeland Security Secretary Michael Chertoff. A bubbling plot by British citizens to blow up airplanes had come to a boil in the past three days, and as British authorities arrested dozens of suspects around London, it was Chertoff's job to coordinate the U.S. defenses. Scary intelligence reports pop up all the time, but this particular terror operation got close enough to being carried out that it rattled even the normally sedate Chertoff. "Very seldom do things get to me," he told Rep. Peter King, the Republican chairman of the House Homeland Security Committee, in a phone call late Wednesday night. "This one has really gotten to me."
Chertoff had good reason to be worried. Senior U.S officials have confirmed to TIME details of the plot that led the Secretary to ratchet up the color-coded security alert for British-U.S. flights to an unprecedented red for "Severe." A total of 24 individuals were arrested in Britain overnight and, says one senior U.S. official who was briefed on the plot, five still remain at large. Their plan was to smuggle the peroxide-based liquid explosive TATP and detonators onto nine different planes from four carriers — British Airways, Continental, United and American — that fly direct routes between the U.K and the U.S. and blow them up in midair. Intelligence officials estimate that about 2,700 people would have perished, according to the official.
Britain's MI-5 intelligence service and Scotland Yard had been tracking the plot for several months, but only in the past two weeks had the plotters' planning begun to crystallize, senior U.S. officials tell TIME. In the two or three days before the arrests, the cell was going operational, and authorities were pressed into action. MI5 and Scotland Yard agents tracked the plotters from the ground, while a knowledgeable American official says U.S. intelligence provided London authorities with intercepts of the group's communications. Most of the suspects are second- or third-generation British citizens of Pakistani descent whose families hailed from war-torn Kashmir. U.S. officials believe the 29 members were divided into multiple cells and planned to break into small groups to board the nine planes.