The UN secretary-general died when the plane carrying him on a mission to bring peace to the Congo crashed in Northern Rhodesia, now Zambia. Commissions established by his native Sweden, the UN, and Northern Rhodesia said pilot error was the most likely cause of the accident.
But a negative can be impossible to prove - and never more so than in the midst of acrisis fuelled by ethnic tension, Cold War rivalries and by commercial interests and their mercenary armies, all vying to control the Congo's richest prize, the breakaway province of Katanga (now Shaba).
Hammarskjold was trying to broker a deal between what passed for a central government in Leopoldville (now Kinshasa) and the Katangan leader, Moise Tshombe, who saw himself as an enlightened African nationalist. Critics saw him as creature of the big mining companies, led by the Belgian Union Miniere, desperate not to lose access to Katanga's copper and diamonds.
Civil war erupted the instant of Belgium's precipitous pull-out in June 1960 from a colony the size of Europe, splintered among 40 ethnic groups. Three men, the first president, Joseph Kasavubu, the future president (then general) Mobutu, and the left-wing prime minister, Patrice Lumumba, vied for control of the country.
At first Lumumba seemed to prevail but he called in Soviet bloc aid to help quell Katanga, which had declared independence. At the height of the Cold War, this was a challenge to the CIA. In January 1961 Lumumba was murdered. The chaos that followed independence had already forced the UN to intervene. By early 1961 the Security Council had authorised the use of force and gradually most of the country was pacified.
Not, however, Katanga, defended by white mercenaries hired by Tshombe, among them "Mad Mike" Hoare. Hammarskjold agreed to mediate. But before he could, he was dead. Or was he killed? And if so, by who? The prime suspects have always been the mercenaries, and the mining companies behind them, desperate to prevent the return of Katanga to central-government control.
In 1992 two former senior UN officials, George Ivan Smith and Conor Cruise O'Brien, said mercenaries sent to seize Hammarskjold accidentally shot down the aircraft just before it landed at Ndola, killing him and 15 others aboard.
Other theories pointed the finger at British and US interests out to prevent UN support for an independent Katanga. If so, and if the letters are to be believed, the South Africans were also in on the scheme.
Amid the confusion and a mystery unlikely to be satisfactorily resolved, one constant endures. Now, as almost four decades ago, the Congo is the real-life African embodiment of Conrad's Heart of Darkness, riven by war and bloodshed, corruption, greed and evil.Reuse content