A United Nations enquiry into the mysterious death of its former head Dag Hammarskjold is due to open.
Mr Hammarskjold died in a plane crash in what is now Zambia in 1961, but the cause of the crash has been disputed.
There have been claims that the plane was shot down, but a previous UN enquiry did not reach a definitive conclusion.
The Swedish diplomat was on his way to negotiate a ceasefire in the breakaway Congolese state of Katanga.
He was trying to mediate between Congo’s Soviet-backed government and Moise Tshombe, who had declared independence for the mineral-rich province of Katanga.
He was flying to Ndola, in what was then Northern Rhodesia, to meet Mr Tshombe, who was backed by former colonial power Belgium and some Western mining interests.
The UN set up this new enquiry after a group of independent investigators said that there was significant new evidence concerning Hammarskjold’s death.
They said the evidence was “persuasive” that “the aircraft was subjected to some form of attack”.
The commission said that the US National Security Agency (NSA) might hold crucial evidence.
It said that given the NSA’s worldwide monitoring activities at that time, “it is highly likely” that the radio traffic on 18-19 September 1961 was recorded by the NSA and possibly also by the CIA.
The report said: “Authenticated recordings of any such cockpit narrative or radio messages, if located, would furnish potentially conclusive evidence of what happened.”
The UN panel will now assess this new information and report back by the end of June.