I see no one culpable in the Trade Center disaster.
I believe the baggage screeners did not miss anything they should have caught. After all, arent' knives with five inch blades allowed.
Pilots are trained to be nonconfrontational when dealing with terrorists and hi-jackers.
I imagine they were.
The last defense could only have been on board marshalls or pilots properly armed.
Unless someone on board is armed and able to stop knife or gun wielders, this could easily happen again despite any increase in security methods and different procedures.