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.
During the past few months the plotters' attack plans had changed, said Deputy Secretary for Homeland Security Michael Jackson. "There were different data sets about their interests over time that evolved," he said. It was only in recent days, said Jackson, that the plans began to focus on British-U.S. flights. The plot was "very near execution" but not imminent, Jackson said. "We didn't pull people off of airplanes."
So as not to derail the British roundup, Chertoff had to wait until the early hours of Thursday morning after all the London arrests were made before notifying U.S. airports of the threat, say senior DHS officals. When it became clear the arrests would be wrapped up around 1 a.m Washington time, Chertoff got on a conference call with his Homeland Security Advisory Committee to approve changing the threat level. Then calls went out to the airlines, airline security companies and labor unions affected by the changes, as well as to members of Congress.
Though the plot has all the hallmarks of an al-Qaeda operation, U.S. officials cautioned that there isn't yet evidence of a direct link between the plotters and the organization's top leaders. "We're not convinced this particular operation is connected to the al-Qaeda chain of command," Charles Allen, Chief of Intelligence for the Department of Homeland Security, told reporters on Thursday afternoon. As for whether the attack was being timed for the fifth anniversary of Sept. 11, Allen said he thought the attack would simply be launched when it was ready. "I am a long-standing believer that terrorist plotters or planners execute when they have all of the plot together," said Allen. "We have no evidence this was timed to any particular holiday or special event."
The plot also appears to be a return to older terrorist tactics of trying to blow up an airplane in midair, rather than turn the jet into a missile as the Sept. 11 attackers did. Allen stressed that the plans seemed designed to kill passengers, not crash into a city on the ground. "We have no evidence there was targeting of cities," said Allen, "This was an effort to destroy multiple aircraft in flight not against any territory of the United States."
With five members of the cell believed to be at large, the threat still looms and intelligence officials are still working to unravel the full extent of the plot. "I don't believe we know all the dimensions of this plot. Time has to pass to determine that a network was disrupted," said Allen. Worries another U.S. official: "Plan A has been stopped, but the concern: Is there a Plan B?"
The possibility that liquid explosives could be smuggled onto a plane is not a surprise to counterterrorism experts, and the tightening of U.S. airport security could only be temporary as security officials learn more about the extent of the plot and how to defend against such an attack. The current measures stripping passengers of anything liquid in their carry-on luggage were in reaction to these particular arrests, and not to the realization of a new, unforeseen threat. "We're primarily concerned about this particular plot," said Allen, implying that the new security measures are not permanent.
FBI and Department of Homeland Security officials quickly alerted law enforcement agencies around the country to the peroxide-based liquid explosives the London plotters planned to bring aboard the American-bound planes. An alert the FBI and DHS sent out Thursday to state and local law enforcement agencies which is classified "For Official Use Only" and was obtained by TIME warns them that the peroxide-based explosives could also be employed in future attacks here.
The Joint Homeland Special Assessment, which the FBI and DHS's Office of Intelligence Analysis drafted and sent out, is titled "Possible Terrorist Use of Liquid Explosive Materials in Future Attacks." The document states: "The FBI and DHS have no information of plotting within the United States, but such a possibility cannot be discounted." The FBI-DHS report notes that Osama bin Laden's top deputy, Ayman al-Zawahiri, insisted in a July 27 videotape that al-Qaeda was still intent on conducting another "spectacular" attack in the United States. Zawahiri, the report notes, used photos of the World Trade Center burning on Sept. 11 and 9/11 leader Muhammad Atta "in the background of this video."
The FBI-DHS report next warns law enforcement agencies about the two peroxide-based liquid explosives that could be used in a future attack against the U.S. triacetone triperoxide (TATP) or hexamethylene triperoxide diamine (HMTD). The report describes how a terrorist would assemble bombs with these chemicals. Peroxide-based liquid explosives "are sensitive to heat, shock, and friction, can be initiated simply with fire or electrical charge, and can also be used to produce improvised detonators," the report states. "For example, TATP or HMTD may be placed in a tube or syringe body in contact with a bare bulb filament, such as that obtained from inside a Christmas tree light bulb, to produce an explosion." The report doesn't mention anything about a terrorist assembling such a bomb on a plane, but it does warn that manufacturing such a device can be dangerous for the bombmaker. "Because of the instability of these substances," the report notes, "spontaneous detonation can occur during the production process."
Over the past ten years peroxide-based explosives have popped up in a number of terror operations, according to FBI-DHS report. "Terrorist have used peroxide-based explosive both as a main charge (weighing in excess of 20 pounds) and improvised detonators," the joint assessment states. "TATP was popularized as a main charge explosive in suicide bombs used by Palestinian terrorist groups."
Ramzi Yousef, who was convicted in 1996 for plotting to simultaneously bomb up to a dozen U.S. commercial airliners flying in the Far East, had manufactured TATP detonators. Arrested Dec. 14, 1999, for planning to attack Los Angeles International Airport in the millennium bombing plot, Ahmed Ressam had HMTD and RDX (cyclotrimethylene trinitrame) in a vial in the trunk of his car. He also had over 100 pounds of urea sulfate white powder and eight ounces of nitroglycerine mixture.
More recently, British shoe bomber Richard Reid tried to detonate his device with TATP as the initiator while aboard a Dec. 22, 2001, American Airlines flight from Paris to Miami. A mixture of TATP and ammonium nitrate was used in suicide bombs in Casablanca, Moroco on May 16, 2003. And the FBI-DHS report notes that four of the suicide bombers in the London subway attack July 7, 2005 "used peroxide-based explosive devices (IEDs), concealed inside rucksacks." With such a rich history, liquid explosives are sure to challenge America's counter-terror defenses for many years to come.