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.
Zambian eyewitnesses told Williams of a bright flash or a second smaller aircraft pursuing Hammerskjold's plane. But one of her most powerful witnesses was thousands of miles (kilometers) away on the night of the crash. In his testimony to the commission, Cmdr. Charles Southall stationed at an NSA eavesdropping post in Cyprus said he heard an intercepted radio conversation apparently from the pursuing plane.
"I've hit it," Southall said he heard the mystery pilot say. "There are flames. It's going down. It's crashing."
The commission's role was not to weigh in on whether Southall's or anyone else's testimony was authoritative. But, in a reference to the NSA's surveillance capabilities, Sedley said it was a "near certainty" that the U.S. was monitoring radio transmissions from the area.
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," he said. The report went on to cite an expert at the archive as saying it was extremely rare for intelligence intercepts to be made public.
The National Security Archive and the National Security Agency did not immediately return messages seeking comment, and any next steps toward a potential investigation aren't clear.
The commission composed of Sedley, Ambassador Hans Corell of Sweden, Justice Richard Goldstone of South Africa, and Judge Wilhelmina Thomassen of the Netherlands has no official standing, although the current U.N. secretary-general or individual member states could ask the General Assembly if they want to pursue its findings.
That could lead to an assembly resolution, which might eventually lead to a formal investigation.
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."
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Edith Lederer at the United Nations contributed to this report.
