I could ask you the exact same question. Do you discount his family and friends?
I have a hard time believing terrorists would go through all of the time and training to take a plane and crash it into a nearly empty building. They seek to maximize their terror, not kill a couple of janitors or somebody working late.
It was fairly close to their rush hour. I think they intended for it to be worse -- they would have been just as satisfied with a financial hit, which started out pretty successful with the downturn in the European market until "accident" was immediately announced.