This is the reverse of what Richard Clarke is claiming. A former FBI translator says the US knew about the 9/11 plans and let it happen anyway:
WASHINGTON, April 2 (UPI) — The U.S. administration knew of al-Qaida plans to target buildings with planes months before the attacks of Sept. 11, 2001, a former FBI translator claims.
Sibel Edmonds, a former translator for the FBI with top-secret security clearance says she spent three hours giving testimony to the panel investigating the attacks, and told Britain’s Independent Friday information was circulating within the FBI in the spring and summer of 2001 an attack using aircraft was months away and the terrorists were in place.
She rejected national security adviser Condoleezza Rice’s claim there was no such knowledge as ‘an outrageous lie.'”
This sounds like the stereotypical Internet conspiracy theory, but other evidence has been floating around for awhile now. Sibel Edmonds is not the only one who believes this.