Widely considered the U.N.'s most effective chief, Hammarskjold died as he was attempting to bring peace to the newly independent Congo. The crash of his DC-6 aircraft in the forest near Ndola Airport in modern-day Zambia has bred a rash of conspiracy theories, many centering on some startling inconsistencies.
For example: 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 riven by Cold War tension. Congo won its freedom from Belgium in 1960, but foreign powers 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. All three nations were jockeying for power with the Soviet Union, which was trying to spread communist influence over the newly independent nations of Africa.
The 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 eyewitness testimony ignored by earlier inquiries.
The four-member commission established to weigh the new evidence was only meant to recommend whether those questions merited further investigation - not answer them. But it suggested that America's most secretive intelligence-gathering agency could be sitting on a key clue.
In his introduction to its report, commission Chairman Stephen Sedley said key evidence may lie with the NSA. Sedley said it was a "near certainty" that the NSA was recording radio transmissions from the African airfield near where Hammarskjold's plane crashed, meaning that American intelligence could help determine whether the U.N. chief died as a result of an accident - or of a conspiracy.
"The only dependable extant record of the radio traffic, if there is one, will so far as we know be the NSA's," he said. "If it exists, it will either confirm or rebut the claim that the DC-6 was fired on or threatened with attack immediately before its descent into the forest."
Sedley said the commission had already sought the help of George Washington University's National Security Archive, a non-governmental research center, in identifying whether the NSA had any relevant information.
"Of three documents or records which appear to respond to our request, two are classified top secret on national security grounds," Sedley said.
The National Security Archive and the National Security Agency did not immediately return messages seeking comment, and what the next step in any potential investigation might be isn't clear.
The current U.N. secretary-general or individual member states could ask the General Assembly if they want to pursue the findings of the commission. This could lead to an assembly resolution, which would give greater international clout to the pursuit of the truth about Hammarskjold's death.
In a statement, the United Nations said it would study the report.
"Secretary-General Dag Hammarskjold gave unparalleled service to the United Nations and paid the ultimate price," the statement said. "The United Nations is among those most concerned in arriving at the whole truth of the circumstances leading to his death."