Commission appeals to US to declassify NSA radio intercepts of warplanes in area where Dag Hammarskjöld's plane crashed

An international panel of retired judges has called for a new United Nations inquiry into the death in 1961 of the UN secretary-general, Dag Hammarskjöld, in an air crash in Zambia.

It says that a significant new evidence has emerged and that the possibility that Hammarskjöld's plane was shot down should "be taken seriously".

The Hammarskjöld Commission – chaired by a former British Privy Council judge, Sir Stephen Sedley, and including the former chief prosecutor at the Hague war crimes tribunal, Sir Richard Goldstone – also appealed to the US to declassify reported National Security Agency (NSA) radio intercepts of warplanes over the area, which are still categorised as top secret 52 years after Hammarskjöld's death.

The commission pointed to several strands of evidence supporting a theory that Hammarskjöld's DC6 airplane was shot down on the night of 17 September 1961, while he was on a mission to Ndola, in what was then Northern Rhodesia, to try to negotiate a ceasefire between the Congolese government and rebels from the mineral-rich Katanga region, who had significant support from Belgian mercenaries.

The report highlighted five key pieces of evidence:

• An apparent confession by a Belgian pilot that he fired on the DC6, known as the Albertina, which he had picked out with twin searchlights mounted under his Fouga jet's fuselage.

• A police sergeant's account of seeing "sparks" in the sky prior to the crash.

• Another eyewitness testimony of seeing "a very bright light" in the sky at the same time.

• A police assistant inspector's testimony that he had seen a flash in the sky.

• The account of a local official, named Timothy Kankasa, of seeing a smaller plane flying above and then alongside the Albertina "shining a beam like a headlight on it".

The commission said it had already requested NSA intercept records from the US national archives in Washington, arguing that "an acid test of the aerial attack hypothesis may be feasible".

"It is a near-certainty not only that Ndola's radio traffic was being monitored routinely by the NSA from Cyprus or elsewhere, but that one or both of the large Usaf aircraft which had been flown in to Ndola on the crucial night and were parked throughout on the tarmac were there for the specific purpose of monitoring the local radio traffic," it added.

A covering letter by Sedley accompanying the commission's report said: Our answer to the question posed to us – whether the UN would be justified in reopening its own inquiry in the light of the evidence now available – is a qualified but firm yes."

He said that a "dependable answer" to the Albertina mystery "is within reach", but that such an answer "will probably not be comprehensive".

It is not clear whether the UN will heed the commission's advice to open a new inquiry. There have already been three investigations, two of which were inconclusive, and one that pointed to pilot error.

A Guardian investigation in August 2011 pointed to the evidence that the Albertina was shot down, and that was followed later in the same year by a book, Who Killed Hammarskjöld?, by British academic, Susan Williams, who argued that there was substantial evidence that hardline Belgian colonialists, outraged at UN support for the Congolese government in Kinshasa, were behind the death of the Swedish UN secretary-general', which was then covered up by the British colonial authorities.

© Guardian News and Media 2013