A chilling, declassified report shows that the FBI suspected bin Laden was planning to hijacked planes, and attack the US:

FBI information since that time indicates patterns of suspicious activity in this country consistent with preparations for hijackings or other types of attacks, including recent surveillance of federal buildings in New York.

It is unfortunate that we did not have enough information to know if and when this was going to happen for sure. At the same time, it is comforting to know that the FBI had some clue before it happened, unlike citizens like us who were so shocked that day.