BS! There were FBI agents who knew about Arabic students taking flight lessons learning how to fly jumbo jets but not land, and there was an FBI informant who was a roomate of one of the 9-11 hijackers. The superiors to the first agent dismissed the importance of the case, and the FBI let the 9-11 hijackers slip away. FBI incompetence and not a wall between LE and intel is what killed 3000 people. The FBI is a lot better at gassing and killing women and children in Waco than protecting national security from known and identified threats.
The wall got in the way on this one.