It was a tasty thread.
Often these days, those losing an argument push some buttons and down the memory hole it goes.
Everyone seems to want a conspiracy... First the pope, now this. As has already been pointed out, the World Trade Center was a known target - that was why there was a security presence there. Aside from putting ground to air missiles up there and someone actually having the guts to light one off into a plane full of bystanders (and risk a miss hitting somewhere in the region), there wasn't much to do.
The only way to prevent it would have been to keep those hijackers from boarding the plane. That wasn't done, what followed was unpreventable. What did we learn? To make sure we strip search granny just about as often as any Arab, to leave our borders open for virtually anyone to cross (and later crash a car into an officer), and to continue ignoring the huge population of those who entered legally but failed to honor their visas and leave under the conditions they agreed to.
Aside from the armed student body, what prevents someone from coming to the US, buying weapons (or making them) and killing our school children? So far as I see it, we're still relying on blind luck.
All that being said, it would be nice to see Graham sweat a bit over this; perhaps we'd have someone else in the Senate to speak up for sane protections for our people.