How did UN Secretary General Dag Hammarskjold die in September, 1961?

Dag Hjalmar Agne Carl Hammarskjold (29 July 1905 – 18 September 1961) was a Swedish diplomat, economist, and author who served as the second Secretary-General of the United Nations, from April 1953 until his death in a plane crash in September 1961. At the age of 47 years upon his appointment, Hammarskjold was the youngest to have held the post. Additionally, he is one of only four people to be awarded a posthumous Nobel Prize and was the only United Nations Secretary-General to die while in office.

He helped coordinate government plans to alleviate the economic problems of the post-World War II period and was a delegate to the Paris conference that established the Marshall Plan. Although Hammarskjold served in a cabinet dominated by the Social Democrats, he never officially joined any political party.

During his term, Hammarskjold tried to smooth relations between Israel and the Arab states. He made a 1955 visit to China to negotiate the release of 11 captured US pilots who had served in the Korean War, the 1956 establishment of the United Nations Emergency Force, and his intervention in the 1956 Suez Crisis.

Hammarskjold was en route to negotiate a cease-fire on 18 September when his Douglas DC-6 airliner crashed with no survivors near Ndola, Northern Rhodesia (now Zambia). Hammarskjold and 15 others perished in the crash, whose circumstances are still unclear. There is some evidence that suggests the plane was shot down.