The NSA gave a release from the NSA caveats to FBI IOS HQ Agent Dina Corsi on August 27, 2001. This release occurred before Corsi told FBI Cole investigator Steve Bongardt that he could not take part in any investigation of Mihdhar because of NSA caveats. But it turns out that there was no legitimate reason FBI Agent Bongardt could not have investigated and searched for Mihdhar once he knew Mihdhar was inside of the US! This investigation would have started almost 2 weeks prior to the attacks on 9/11 and would almost certainly have prevented the attacks on 9/11.
Verifying documentation: NSA approves sharing info
See the book "Prior Knowledge of 9/11" for more in depth information on this.