On september 11, 2001, they watched two planes hit the world trade center. >> we knew, immediately, as soon as we saw the images, that we needed to protect and defend. >> o'donnell: protect and defend the nation's capitol, as commercial airliners had become weapons of war. what did you understand about the rules of engagement, heather? >> we understood what the threat was. we were looking for a rogue airliner, flying low, that was not communicating with air traffic control. >> o'donnell: the pilots had no time to spare. the country was under attack. they knew of at least one plane still in the...