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Following the events of September 11, 2001, airport security screening history changed drastically. When terrorists were able to hijack four passenger airliners and drive three of them into large structures, killing thousands, governments around the world responded with new procedures. In the United States, this resulted in the establishment of the Transportation Security Administration (TSA), a government agency that is responsible for all screenings of passengers at airports.
New rules were created about what could and could not be brought aboard airplanes. Box cutters and certain other objects that could previously be carried on board were now subject to confiscation. In addition, the newly-created Department of Homeland Security instituted the policy of secondary screenings of passengers and a No-Fly List of individuals in the country and around the world that would not be authorized to fly aboard airplanes.
Above left: September 11, 2001 Trade Center Collapse. (Supplied by Wally Gobetz at Wikimedia Commons; Creative Commons; http://upload.wikimedia.org/wikipedia/commons/5/55/September_11_2001_just_collapsed.jpg)