by Shoestring source: 9/11 Blogger April 11, 2011
“There were a number of false reports out there. What was valid? What was a guess? We just didn’t know.”
– Colonel Robert Marr, battle commander at
NORAD’s Northeast Air Defense Sector on 9/11
Although it has been widely reported that four commercial aircraft were hijacked over the United States on September 11, 2001, what is less well known is that while the terrorist attacks were taking place and for many hours after, numerous additional aircraft gave indications that they had been hijacked or, for other reasons, were singled out as potential emergencies. More than 20 aircraft were identified as possible hijackings, according to some accounts, and other aircraft displayed signs of emergencies, such as losing radio communication with air traffic controllers or transmitting a distress signal.
Reports about these false alarms have revealed extraordinary circumstances around some of the incidents and bizarre explanations for how they arose. For example, it has been claimed that the pilots of one foreign aircraft approaching the U.S. set their plane’s transponder to transmit a code signaling they had been hijacked simply to show authorities that they were aware of what had been taking place in America that morning.  Another aircraft reported as transmitting a distress signal while approaching the U.S. was subsequently found to have been canceled, and still at the airport. 
There may be innocent explanations for some of the less serious false alarms, such as those simply involving the temporary loss of radio communication with the plane, which is a common occurrence and happens on a daily basis.  But, viewed in its entirety, the evidence appears highly suspicious and raises serious questions. Why, for example, were there so many false alarms on September 11? Why did so many of them involve false reports of hijackings or aircraft falsely signaling that they had been hijacked? The details of specific incidents that have been reported, which I describe below, show that these false alarms must have been something more than just the results of confusion caused by the terrorist attacks.
MILITARY EXERCISES INCLUDED SIMULATED HIJACKINGS
One possibility to consider is that some of the false alarms related to training exercises taking place on September 11. There is evidence supporting this contention. For example, shortly after 9/11, the New Yorker reported, “During the last several years, the government regularly planned for and simulated terrorist attacks, including scenarios that involved multiple plane hijackings.”  And we know that the North American Aerospace Defense Command (NORAD), the military organization responsible for defending U.S. airspace, was in the middle of a major exercise called Vigilant Guardian on September 11.  This exercise is known to have been scheduled to include at least one simulated plane hijacking on the morning of 9/11.  And in the week before 9/11, it included at least four simulated plane hijackings.