What was the pre-9/11 A l-Qa ida strength & what did the U S know pre-911 & if they knew why didnt they act?

Our government was informed of an impending attack by a group of fanatics but felt that no one would ever follow though with such an action against one of the most powerful nations in the world. America had never dealt with terrorists on such a scale before, they couldn't accept such an idea as being probable. The government didn't know just what to do or where to start to stop this type of fighting, we have learned, lets hope we have learned well.