On 17 September 1961, UN Secretary General Dag Hammarskjöld was flying to Rhodesia to negotiate peace talks between the government and Tshombe when Hammarskjöld's plane crashed, killing him. It was a response to the troubled Operation Morthor ongoing at the time. The circumstances of his death were suspicious, and there remains suspicion that his plane was shot down. A recent book by Susan Williams (2011) uncovered evidence that a specially equipped fighter plane from the Belgian-led mercenary force referred to as the Katanga Gendarmerie shot down his plane. Hammarskjöld was succeeded by U Thant.