If you ask me, any subsequent attacks will be of a totally different nature. No terrorist organization intelligent enough to conduct a plan like that is dumb enough to try the same thing twice. Expect the unexpected, because they'll try to find another way, believe me. The explosives etc. Linus mentioned were presumably back up, in case anything went wrong. But I heard a story of a man that jumped off the WTC, and landed safely on a shop awning cover. I don't know whether he survived the subsequent events. But I just think it is amazing that that happened.
