The then Secretary General of the United Nations, Dag Hammarskjöld, died during the night of September 17th 1961 in a plane crash in what is now Zambia, where he was headed to mediate in the ongoing conflict in neighbouring The Congo.
The diplomat's death has been the subject of numerous rumours and conspiracy theories over the past five decades centred around whether the crash was an accident, or if Hammarskjöld was killed.
Evidence available has left investigators puzzled, with pilot error deemed unlikely after witnesses claimed to have seen the plane going down on fire.
Investigators who probed the case urged the United Nations on Monday to launch a new investigation into the crash, stating that the possibility that the plane was attacked from above, or that it was forced down due to threats, should be "taken seriously, despite everything".
The team of investigators, which was led by four senior lawyers including diplomat Hans Corell, appealed to the United States to declassify documents from the National Security Agency (NSA) including radio communications and intercepts of war planes in the area at the time.
The commission added that it was a "near certainty" that all air traffic information around the airport was "followed and recorded by the NSA and possibly even the CIA". Access to such files has been denied by the NSA due to the "top secret" classification, something the commission wants to be lifted to further the investigation.
A recent book by the author Susan Williams entitled Who Killed Hammarskjöld? also argued that the plane was brought down, and prompted the diplomat's nephew Knut Hammarskjöld to call for the new inquiry.