A perfect example of Monday Morning quarterbacking.
I doubt if there is/was a single person in the U.S. who would have thought of an operation of the type and scope of 9/11. Of course,
NOW everyone does. The indications were hijacking. Until this event hijackers had other purposes in mind than suicide. The procedure was to safely talk/negotiate the plane to the ground.
The FBI blew it but I don't believe anyone would have acted differently
BEFORE the incident.
Honestly think of what you would say if
prior to 911 someone told you that four airliners would be simultaneously taken over and flown into the WTC and the Pentagon. I would have probably have asked when the movie was coming out.
Our intelligence needed/needs revamping but give credit to a crafty enemy who thinks up totally new and unexpected tactics. OBL wanted to bring down the WTC so bad that this was his second attempt.