Military personnel responsible for defending U.S. airspace had false tracks displayed on their radar screens throughout the entire duration of the 9/11 attacks, as part of the simulation for a training exercise being conducted that day. Technicians at NORAD's Northeast Air Defense Sector (NEADS) were still receiving the simulated radar information around the time the third attack, on the Pentagon, took place. Those at NORAD's operations center in Cheyenne Mountain, Colorado, were still receiving it several minutes after United Airlines Flight 93 apparently crashed in rural Pennsylvania.
No one has investigated why false tracks continued being injected onto NORAD radar screens long after the U.S. military was alerted to the real-world crisis taking place that morning. And yet we surely need to know more about these simulated "inputs" and what effect they had on the military's ability to respond to the 9/11 attacks.