Complex sentence: Occasionally, however, the hijackers have flown the aircraft themselves, such as the September 11 attacks, and in at least three cases, the plane was hijacked by the official pilot or co-pilot.
Split into simpler sentences: 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.