Heck, a repeat of 9-11 went out the door when they decided to lock the cockpit. Which, really, they should have done much earlier considering all the planes that were hijacked in the past and the previous attempt(s?) to use planes as weapons against buildings. I can think of at least one person who got as far as a plane, intending to have it flown into the White House, I believe, long before 9-11.
Really shoulda locked those doors....