On the evening of September 17, 1961, the DC-6 airplane carrying Hammarskjöld crashed before it was supposed to land in Ndole in Rhodesia, now in Zimbabwe.
Since then, speculative theories as to the cause of the crash have abounded, ranging from mechnical failure to sabotage, but a UN report from 1962 dismissed any theories related to sabotage.
Göran Björkdahl, who works for the Swedish International Development Cooperation Agency (Sida) in Burkina Faso, has been researching what happened to Hammarskjöld in his free time. For three years when Björkdahl was working in Zambia, formerly Northern Rhodesia, he gathered 10 witness testimonies that all point to the same conclusion: Hammarsjöld's plane was shot down by a smaller aircraft.
"That is the conclusion one must draw. It seems likely that he was murdered, based on the testimonies that have come to light," said Björkdahl, speaking with Swedish Radio News.
"They have seen this DC-6 come in. Some say the plane circled two or three times. Several of them say they saw a smaller plane approach the DC-6 from behind and then shoot something down, something glowing toward the plane. They saw an explosion on the front top part of the plane before it crashed," he said.
Björkdahl said the witnesses he has spoken with have seen the same thing from different positions, some from far away, others from a closer distance. "The accounts are largely the same," he said.