I remember it being said by some government guy on some 9/11 documentary I saw that their was either a drill done for a scenario almost exactly like what happened, or the guy who thinks up ideas that terrorist might try so we can find ways to stop it happening proposed that terrorists might try something like 9/11, but they said it was unlikely, and it's not a threat to worry about too much. I forgot which it was, but the gist of it was that the government had at some point been aware that a 9/11 type scenario could theoretically happen. That said, being aware that something is possible doesn't mean a whole lot when it's buried in a mountain of other possible things terrorists might do, and there's no way to prepare for all of those possibilities.