I want to expand a bit on what I posted yesterday, just to be certain the point is clear. In the Old Testament, the prophet Amos tells us that God said, "does disaster befall a city unless I have done it?" He is showing that nothing happens outside of the sovereign will of God. No, God does not always bring about disaster actively; it sometimes happens merely because He passively let it happen. We cannot, however, presume that a disaster happens out of His control. He is either active or passive in it. What was He in the 9/11 disaster in America? Only He knows, but He was not caught off guard or out of control (that is a pagan view of God).
Therefore, whatever God was thinking about during the 9/11 attacks (discipline, judgment, consequences of actions, etc.), He definitely, at the barest minimum, allowed them to happen. Thus, there is a lesson that can be learned from them. In fact, every trial and disaster is an opportunity for us to repent and come closer to God. This one was no different in that way. Going back further, did we learn a lesson from the Pearl Harbor attacks? It does not matter if our sins were the cause of the event or not, pain is one of the most important means of God leading us to righteousness (if we respond to it righteously!).
Therefore, with that added detail, I will ask it again: have we learned our lesson?