As KO noted on his show, the US picked up the chatter and made the decision to crack down on the plot today. Yeah, they picked up the chatter...
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http://www.time.com/time/nation/article/0,8599,1225453,00.html<snip>
Exclusive: U.S. picked up the suspects' chatter and shared it with British authorities; new federal alert warns that peroxide-based explosives could also be employed in future attacks in the U.S.
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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 round-up, 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 when out to the airlines, airline security companies and labor unions affected by the changes, as well as to members of Congress.