pages 140 and 141 of Al Gore's Assault on Reason:
In late August 2001, Nawaf Alhazmi and Khalid Almidhar bought tickets to fly on American Airlines Flight 77 (which was flown into the Pentagon). They bought the tickets with their real names. Both names were then on a State Department/INS watch list called TIPOFF. Both men were sought by the FBI and CIA as suspected terrorists, in part because they were observed at a terrorist meeting in Malaysia.
These two passenger names would have been exact matches when checked against the TIPOFF list. But that would only have been the first step. Further data checks could then have begun.
Checking for common addresses (address information is widely available, including on the Internet), analysts would have discovered that Salem Alhazmi (who also bought a seat on American Airlines Flight 77) used the same address as Nawaf Alhazmi. More important, they could have discovered that Mahomed Atta (American Airlines Flight 11, North Tower of the World Trade Center) and Marwan Al-Shehhi (United Airlines Flight 175, South Tower of the World Trade Center) used the same address as Khalid Almidhar.
9/11 could have been stopped with existing pre-9/11 laws and simple police work.
Gore lists his source as
http://www.markle.org/downloadable_assets/nstf_part_1.p... and the 9/11 Commission Report backs it up.
Repost from one I did in June:
http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=389&topic_id=1124326