It's not that Norad and the FAA didn't respond that day, they couldn't:

http://valis.cjb.cc/index.php?name=News&file=article&sid=97

PROMIS Connections to Cheney Control of 9/11 Attacks Confirmed

* The FAA & Ptech
* Debriefed by Secret Service - looking for a PROMIS
* Muslim Brotherhood, Christian Cultists, and Nazis
...
Whoever plotted 9/11 definitely viewed the FAA as an enemy that morning. Overriding FAA systems would be the most effective way to ensure the attacks were successful. To do this, the FAA needed an evolution of PROMIS software installed on their systems and Ptech was just that; the White House & Secret Service had the same software on their systems - likely a superior modified version capable of "surveillance and intervention" functions.

It has been clearly documented that "false blips," or radar injects, were placed on FAA radar screens on the morning of 9/11 as part of the Air Force war games that morning. 2 "Phantom flight 11" fits the description of a "false blip." If it was, in fact, a radar inject, that would explain why the 9/11 commission was unable to locate the source of the "mistaken FAA information." The 9/11 war games are classified and specific information regarding such details is not publicly available. We do know by the time "phantom flight 11" appeared on FAA screens - 9:24 am - the war games had reportedly been called off.