I didn't know this at all, so I have to admit that I did cheat and look this one up.
"In 1961, the prize was awarded to the second Secretary General of the UN, Dag Hammarskjöld, for strengthening the organization. Hammarskjöld is the only person to have received the prize posthumously, a few months after his death in a plane crash in the Congo; the Nobel statutes were later changed to make a posthumous prize virtually impossible."
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