The U.N. leader died in a plane crash in 1961. Many suspect foul play, but a lot of information still remains secret. Dag Hammarskjöld, the secretary general of the United Nations, in London in the mid-1950s. (Photo: Topical Press Agency/Getty Images) For a long time, the 1961 plane crash that killed the U.N. secretary general Dag Hammarskjöld has been a mystery: a nighttime catastrophe in Cold War Africa, unexplained and unexplainable. In recent years, United Nations investi