Four American special operations personnel were killed in an airplane crash in Djibouti over the weekend, the U.S. military said Monday. The aircraft was a U-28, a modified single engine turbo prop plane used by special operations units. It crashed Saturday near a base used by the U.S. military at Camp Lemonnier. The U.S. Africa Command said the plane was on a routine flight when it went down 10 kilometers from the Djibouti International Airport. The cause of the crash is under investigation.