Airlines security policies post 9/11
Box-cutter knives were used in the September 11, 2001 attacks. The hijackers were allowed to pass security with these able to carry these knives because at the time US domestic flights permitted any knife with a blade up to 4 inches (100 mm) long.
Federal Aviation Administration rules prohibiting any type of knife in secured airport areas and on airplanes was put into effect on September 13, 2001.
TSA has a list of items that are banned from bringing aboard.