Hammerskjold's aircraft went down on the night of Sept. 17, 1961, smashing into a forested area just short of Ndola Airport in modern-day Zambia. A host of hard-to-answer questions about the crash have led to a glut of conspiracy theories.

Among them: Why did it take 15 hours to find the wreckage, just a few miles from the airport? Why did Hammarskjold's bodyguard, who survived the crash for a few days, say that the plane "blew up"? Why did witnesses report seeing sparks, flashes, or even another plane?

Hammarskjold was flying into a war zone infested with mercenaries and riven by Cold War tension. Congo won its freedom from Belgium in 1960, but foreign multinationals coveted its vast mineral wealth and the country was challenged by a Western-backed insurgency in Katanga, which hosted mining interests belonging to United States, Britain, and Belgium. They were also jockeying for influence with the Soviet Union, which was trying to spread communism to the newly independent nations of Africa.

All four powers had a stake in the outcome of Congo's struggle, and all four have been fingered as potential suspects in Hammarskjold's death.

Three investigations into the tragedy have failed to satisfactorily settle the matter, and the publication of "Who Killed Hammarskjold?" by Susan Williams in 2011 set off a renewed round of speculation — not least because of its reliance on testimony ignored by earlier inquiries.