because placing demolitions in the most effective places is OBVIOUS. It requires mass amounts of plastic explosives. The most effective placement would be in the parking garage, which no one could miss. I fail to see how flying a plane into a building could have been a warning in any way, since it hasn't happened before in recorded history, at least, not on purpose. We didn't know it was al Qaeda. It could easily have been an accident, at least until the second plane hit. I remember watching the news broadcast and wondering what the fuck was going on quite clearly. It was only tentatively al Qaeda around twelve hours later.