Dag Hjalmar Agne Carl Hammarskjöld (; 29 July 1905 – 18 September 1961) was a Swedish diplomat, economist, and author. The second Secretary-General of the United Nations, he served from April 1953 until his death in a plane crash in September 1961. At the age of , Hammarskjöld is the youngest to have held the post. He is one of only four people to be awarded a posthumous Nobel Prize. Hammarskjöld is the only UN Secretary-General to die in office; he was killed in a Douglas DC-6 airplaneen route to cease-fire negotiations. US PresidentJohn F. Kennedy called Hammarskjöld "the greatest statesman of our century".