BBC News - 9/11: Clinton says Al-Qaeda behind New York threat. "It's accepted wisdom today among counter-terrorism officers that the best defence against terrorist attack is an alert citizen. The shoe bomber Richard Reid, and the Christmas plane bomber were both foiled by ordinary people, and that explains why the authorities have moved so aggressively to alert Americans to this possible threat." said Adam Brookes, BBC News, Washington

BBC News - 9/11: Clinton says Al-Qaeda behind New York threat
"She said the decision to go public with the threat was "intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and ears of vigilance", adding that the threat was being taken very seriously by state and federal authorities."

I have sent these to the Shanksville, PA Post Office for philatelic postmarking, to create maximum cards.
Notice the flight path of US Airways, Flight 93.
"Let's roll!"