On 9/11, the terrorists turned off the transponders on three of the four hijacked aircraft. With its transponder off, it is possible, though more difficult, to track an aircraft by its primary radar returns. But unlike transponder data, primary radar returns do not show the aircraft's identity and altitude. Controllers at centers rely so heavily on transponder signals that they usually do not display primary radar returns on their radar scopes. But they can change the configuration of their scopes so they can see primary radar returns. They did this on 9/11 when the transponder signals for three of the aircraft disappeared.⁹⁴

Before 9/11, it was not unheard of for a commercial aircraft to deviate slightly from its course, or for an FAA controller to lose radio contact with a pilot for a short period of time. A controller could also briefly lose a commercial aircraft's transponder signal, although this happened much less frequently. However, the simultaneous loss of radio and transponder signal would be a rare and alarming occurrence, and would normally indicate a catastrophic system failure or an aircraft crash. In all of these instances, the job of the controller was to reach out to the aircraft, the parent company of the aircraft, and other planes in the vicinity in an attempt to reestablish communications and set the aircraft back on course. Alarm bells would not start ringing until these efforts-which could take five minutes or more-were tried and had failed.⁹⁵

NORAD Mission and Structure. NORAD is a binational command established in 1958 between the United States and Canada. Its mission was, and is, to defend the airspace of North America and protect the continent. That mission does not distinguish between internal and external threats; but because NORAD was created to counter the Soviet threat, it came to define its job as defending against external attacks.⁹⁶

The threat of Soviet bombers diminished significantly as the Cold War ended, and the number of NORAD alert sites was reduced from its Cold War high of 26. Some within the Pentagon argued in the 1990s that the alert sites should be eliminated entirely. In an effort to preserve their mission, members of the air defense community advocated the importance of air sovereignty against emerging "asymmetric threats" to the United States: drug smuggling, "non-state and state-sponsored terrorists," and the proliferation of weapons of mass destruction and ballistic missile technology.⁹⁷

NORAD perceived the dominant threat to be from cruise missiles. Other threats were identified during the late 1990s, including terrorists' use of aircraft as weapons. Exercises were conducted to counter this threat, but they were not based on actual intelligence. In most instances, the main concern was the use of such aircraft to deliver weapons of mass destruction.

Prior to 9/11, it was understood that an order to shoot down a commercial aircraft would have to be issued by the National Command Authority (a phrase used to describe the president and secretary of defense). Exercise planners also assumed that the aircraft would originate from outside the United States, allowing time to identify the target and scramble interceptors. The threat of terrorists hijacking commercial airliners within the United States-and using them as guided missiles-was not recognized by NORAD before 9/11.⁹⁸

Notwithstanding the identification of these emerging threats, by 9/11 there were only seven alert sites left in the United States, each with two fighter aircraft on alert. This led some NORAD commanders to worry that NORAD was not postured adequately to protect the United States.⁹⁹

In the United States, NORAD is divided into three sectors. On 9/11, all the hijacked aircraft were in NORAD's Northeast Air Defense Sector (also known as NEADS), which is based in Rome, New York. That morning NEADS could call on two alert sites, each with one pair of ready fighters: Otis Air National Guard Base in Cape Cod, Massachusetts, and Langley Air Force Base in Hampton, Virginia.¹⁰⁰ Other facilities, not on "alert," would need time to arm the fighters and organize crews.

NEADS reported to the Continental U.S. NORAD Region (CONR) headquarters, in Panama City, Florida, which in turn reported to NORAD headquarters, in Colorado Springs, Colorado.