http://www2.fbi.gov/pressrel/penttbom/penttbomb.htm
Excerpt:
They were the most lethal terrorist attacks in history, taking the lives of 3,000 Americans and international citizens and ultimately leading to far-reaching changes in anti-terror approaches and operations in the U.S. and around the globe.
Our ensuing investigation of the attacks of 9/11/01—code-named “PENTTBOM” —was our largest investigation ever. At the peak of the case, more than half our agents worked to identify the hijackers and their sponsors and, with other agencies, to head off any possible future attacks. We followed more than half-a-million investigative leads, including several hundred thousand tips from the public. The attack and crash sites also represented the largest crime scenes in FBI history.
