They probably pushed the panic button today because their surveillance caught some men shaving their bodies and going to mosque at an unusual time.
LOL! Well, here's a clue from the always anonymous government source in the WaPo:
"This wasn't supposed to happen today," said a U.S. official who asked not to be identified. "It was supposed to happen several days from now. We hear the British lost track of one or two guys. They had to move. "
So 9/11 could have been the target date all along (those jihadis do love their anniversaries), but if the Brits "lost track" of one or two of the terrorists, the air travel mess may indicate they haven't found them yet.