For more than half a century, the circumstances of the tragic death of Dag Hammarskjöld have been shrouded in mystery.

That the former UN secretary-general died in a plane crash while on his way to negotiate a ceasefire in the breakaway African republic of Katanga, is well documented. But the cause of the crash remains to be established.

That proof may become available if the US National Security Agency (NSA) complies with a new request from the UN and hands over crucial intelligence intercepts that could confirm what brought down the Albertina DC6 in a forest near Ndola in northern Rhodesia (now Zambia) in September 1961. All but one on board died in the crash. A 16th passenger, Sergeant Harold Julien, the acting chief security officer, died as a result of injuries a few days later, having told medical staff he had seen “sparks in the sky” shortly before the crash.

Three separate inquiries have been unable to come to a definitive conclusion about what happened on the fateful night. Since then, conspiracy theorists have gone into overdrive, possibly with good cause. The day after the crash, former US president Harry Truman told reporters Hammarskjöld “was on the point of getting something done when they killed him. Notice that I said ‘when they killed him’.” Truman refused to elaborate but this served only to fuel the rumour mill.

Many countries had an interest in thwarting Hammarskjöld’s attempts to reunite Congo and stop Katanga seceding. Congo, which was receiving Soviet aid, had the world’s richest uranium resources. Mining firms feared their concessions would be jeopardised if Katanga’s bid for independence was not recognised. The KGB, the CIA and MI6 were all active in the country as they sought to secure their countries’ interests.

A report from a panel of distinguished international jurists, commissioned by the Hammarskjöld Inquiry Trust, chaired by Lord Lea of Crondall and submitted to the UN last year, heard suggestions “that a group representing a number of European political and business interests ... wanted the secretary-general’s plane diverted from Ndola ... in order to persuade him of the case for Katanga’s continued independence”.

Quite how forcefully such interests may have made their case could explain the cause of the crash. Several people who attended the crash scene claimed the Albertina was riddled with bullets. A delay of more than nine hours in locating the plane has triggered speculation that the crash site was interfered with. Several witnesses said they saw between six and eight white men, armed and in combat fatigues, at the crash site.

Then there are lurid claims that some on board, including Hammarskjöld himself, had bullet wounds, suggesting he had survived the crash but had been killed on the spot. His body was found propped up on a termite mound a short distance from the plane, surrounded by playing cards. Reputedly an ace of spades was found on his collar.

In the aftermath, loose-lipped mercenaries claimed that they had played a part in the operation. In 1998 the South African truth and reconciliation commission published intelligence documents suggesting that a bomb had been placed on board. A separate CIA report implicated the KGB.

Given so many competing claims, some doubt whether the truth will ever surface. But tomorrow the UN will signal that the investigation into what happened should be reopened, when it is expected to pass a motion asking member states to provide it with any information they have on the affair.

The resolution is more than a symbolic gesture. It is believed that the US National Security Agency is in possession of radio intercepts, captured by its listening station in Cyprus, that will prove or disprove whether a second plane was in the air near Ndola shortly before the crash. If so, this would help corroborate claims made by a former Belgian pilot called Beukels that he accidentally shot down the Albertina in an attempt to divert it from Ndola to another airport.

Facebook Twitter Pinterest Dag Hammarskjold died in a plane crash in 1961.

Beukels said he shone a bright light down from his Fouga jet onto the cabin of the DC6 while his radio operator called on the plane to divert. After failing to receive a reply, Beukels fired several warning shots which clipped the Albertina’s wing, causing it to crash.

The report commissioned by Lea suggested that the NSA’s intercepts could be the “golden thread” that finally unravels the mystery. It stated that “it is highly likely that the entirety of the local and regional Ndola radio traffic on the night of 17-18 September 1961 was tracked and recorded by the NSA, and possibly also by the CIA”. In his letter introducing the report to the UN, Lea observed that “there is persuasive evidence that the Hammarskjöld aircraft was subject to some form of attack as it circled Ndola”.

So far requests to the NSA to produce relevant intercepts have been denied. But a direct appeal from the UN may cause the agency to think again. As the report notes: “It is thus possible that the last half-century, far from obscuring the facts, may have brought us somewhat closer to the truth about an event of global significance which deserves the attention both of history and of justice.”