America's National Security Agency may hold crucial evidence about the cause of the 1961 plane crash which killed United Nations Secretary-General Dag Hammarskjold, one of the greatest unsolved mysteries of the Cold War.
A commission established to see if the case merited further investigation said key evidence may lie with America's electronic eavesdropping agency,
Widely considered the UN's most effective chief, Mr Hammarskjold died as he was attempting to broker a ceasefire in the newly independent Congo. The crash of his DC-6 aircraft near Ndola Airport in modern-day Zambia has bred a rash of conspiracy theories, many centring 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 Mr 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?
Was Mr Hammarskjold assassinated, perhaps as the result of colonial greed or Cold War rivalry?
In his introduction to its report, commission chairman Stephen Sedley it was a "near certainty" that the NSA was recording radio transmissions from the African airfield near where Mr Hammarskjold's plane crashed, meaning that American intelligence could help determine whether the UN 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."
Mr Sedley said that the commission had already sought the help of George Washington University's National Security Archive, a non-governmental research centre, 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.