There was a fifth hijack plane, with 4 intented hijackers on board. The potential hijackers never did anything, and the plane landed safely in Dallas (I forget its origin). Since the four on that plane never committed an overt act, they were allowed to deplane. As I recall, one of the four was female, and at least three were Saudi. Beyond that, the Internet has swallowed any details (NSA/CIA/FBI).
Imagine that.