In September 1961 Secretary-General of the UN Dag Hammarskjold, was killed in a plane crash in Northern Rhodesia (now Zambia). Hammarskjold was flying to negotiate a cease-fire in the Congo where a rebellion was openly being backed by Belgium and secretly by Britain and France.

The circumstances of the crash are still unclear. A 1962 Rhodesian inquiry concluded that pilot error was to blame, while a later UN investigation could not determine the cause of the crash. Some evidence suggested the plane was shot down. A US  Central Intelligence Agency report claimed the Soviet KGB was responsible.

Source: Wikipedia