Of course the FBI had such reports, but still they didn't believe it could happen in the United States. The FAA also came to that same conclusion as well, and the rest is history.
We also had warnings that dated back to 1995.
And still, they dropped the ball. Remember, we had warnings of Pearl Harbor, but there were those who didn't think it could happen, however, that Japanese attack on December 7, 1941 made believers out of skeptics.