Aircraft hijacking (also known as air piracy or aircraft piracy, especially within the special aircraft jurisdiction of the United States, and informally as skyjacking) is the unlawful seizure of an aircraft by an individual or a group.[https://www.law.cornell.edu/uscode/text/49/46502] In most cases, the pilot is forced to fly according to the orders of the hijackers. Occasionally, however, the hijackers have flown the aircraft themselves, such as the September 11 attacks. In at least three cases, the plane was hijacked by the official pilot or co-pilot.