On the evening of September 17, 1961, a plane fell out of the sky in British-ruled Northern Rhodesia, known today as Zambia. According to meteorology reports, the night was clear and calm.

On board the DC-6 airliner was the 56-year-old United Nations Secretary-General Dag Hammarskjöld — who was on his way, under the cover of night, to negotiate an urgent ceasefire.

The warring parties were many and varied. On the one side was the then Republic of Congo, which had just declared independence from Belgium, and its anti-colonial supporters. On the other side were secessionist forces in the province of Katanga — backed by mining executives, reams of mercenaries, and some high-up officials within American and European governments, all of whom were fighting to see Africa's colonial order maintained. Katanga had become a post-colonial maelstrom.

The stakes were high: in no small part because masses of radioactive uranium had been found in Katanga. Indeed, uranium from Katanga was used in the nuke that American forces dropped on Hiroshima. In 1961, in the thick of the atomic Cold War, the American government was desperate to prevent the Soviets from getting any of it.

But back to Secretary-General Hammarskjöld. His plane never landed. It crashed in the thick bushlands near Ndola and the youngest-ever UN secretary-general died.

In 1961 and 1962, the Rhodesian government claimed that Hammarskjöld and his fellow passengers had perished on impact in a crash caused by pilot error. But a 1962 UN inquiry reached an open verdict — and would not rule out sabotage.

All along, eyewitnesses disputed the official version of events. Several locals reported seeing two aircraft in the sky before the crash, with light or fire transferring from the smaller plane to the larger plane. Others claimed that they had visited the crash site and seen bullet holes in the aircraft.

And so opened one of the great, unsolved mysteries of the Cold War era.

Last week, the United Nations published a much-anticipated inquiry into the crash and the deaths of 16 passengers. In the report, UN investigators said that they had found "significant new information" supporting the theory that Hammarskjöld's plane was downed by "aerial attack or other interference."

The report followed months of investigative work. A three-person "Independent Panel of Experts" combed through historic archives and interviewed handfuls of experts, and traveled to Zambia to speak with 12 living eyewitnesses to the crash. The report offered no conclusive findings, but it did cast doubt on official versions of events — to the delight of conspiracy-minded Hammarskjöld watchers.

In a letter dated July 2, current UN Secretary-General Ban Ki-moon personally recommended a further investigation into the incident: "This may be our last change to find the truth."

The leading theory is that Hammarskjöld's plane was shot down, by opponents of his anti-colonial stance in Africa. Perhaps the shooters were mercenaries fighting for Katanga on behalf of Belgium. Or perhaps, they were employees of rich European mining companies that had lucrative stakes in the Katanga province and feared losing their concessions.

Other think the crash was the product of great power machinations. In 2013, an international commission of distinguished jurists, chaired by the Hammarskjöld Inquiry Trust, reviewed evidence "that a group representing a number of European political and business interests [had] wanted the secretary-general's plane diverted from Ndola... in order to persuade him of the case for Katanga's continued independence."

1961, after all, was a time of continental reinvention that had been long anticipated, but bitterly resisted, by Europe's moribund colonial powers. Everyone wanted a piece of the post-colonial break-up and parts of Africa, including the Congo, had become proxy battlegrounds for larger, Cold War disputes. Hammarskjöld, who supported independence for African states, was making life difficult for Europe's collapsing colonies.

But absolute truth may prove illusive in this instance, at least for Mr. Ban. A number of UN member states — including the US, UK, Belgium, and South Africa — will not play ball with UN investigators. Namely, these member states have refused to hand over historic government documents, which could shed light on the long-ago crash.

The US has already been singled out by critics who allege that it is covering up evidence about the CIA and NSA's former involvement with Katanga secessionists.

Thus far, the UK has largely escaped scrutiny — despite the fact that it too has refused to hand over uncensored state files.

This is troubling to UN investigators, especially given that newly discovered historical documents place at least one British intelligence agent in the Ndola area at the time that Hammarskjöld's plane came down.

On April 27, the UN Hammarskjöld's panel wrote to the UK Foreign & Commonwealth Office (FCO), asking whether the British government had any undisclosed information that might aid its investigation.

On June 10, two days before the panel concluded its work, the FCO replied with a sparsely worded email, explaining that it had "co-ordinated a search across all relevant UK departments. None of these departments has identified any pertinent material."

But what does "relevant" mean?

And, importantly, were the British intelligence agencies MI5, MI6, and UK Government Communications Headquarters (GCHQ) included in the "search"?

VICE News contacted the FCO, asking for further clarification. But an FCO spokesperson would say only: "It is the longstanding policy of successive British governments not to comment on intelligence matters." The UK Home Office also refused to comment, referring VICE News back to the FCO.

The thing is, nobody in Britain comments on intelligence matters.

In the UK, intelligence and security agencies do not fall under the Freedom of Information Act — so it is impossible to compel the agencies to release information to the public. By contrast, the American CIA, FBI, and NSA can be subject to Freedom of Information requests. In fact, MI5 and MI6 do not even have press offices that can answer queries from journalists.

VICE News also contacted the GCHQ, which would not comment on whether it had been included in the Hammarskjöld's panel search. But the department did invite VICE News to submit a Freedom of Information request on the matter, so that GCHQ could formally decline the Freedom of Information request, on the grounds that GCHQ is not obliged to respond to Freedom of Information requests.

Among the British documents that are of interest to UN officials is a report from a British High Commission official and MI6 official Neil Ritchie, dated September 17, 1961. In the report, Ritchie describes how he personally transported a Katanga leader to Ndola to meet with Hammarskjöld.

The document, which was discovered by historian Dr. Susan Williams in a private archive at the University of Essex, does not reveal the cause of the crash — but it does prove that UK intelligence was working in the area at the time and keeping records of its work.

The UN panel was also interested in six slim files that are held at the UK National Archives, which have been publicly released, but with redactions. One is titled: "Enquiry into circumstances of crash of aircraft carrying Dag Hammarskj__öld, UN secretary-general — 1962." Another is titled: "Activities of mercenaries in Belgian Congo — 1961."

The UN asked for uncensored versions of the documents, but the FCO said declined: "For security-related reasons."

At the same time, Foreign Office officials insisted that "the total amount of information withheld is very small."

In 1960 — one year after nationalist riots swept Leopoldville (now Kinshasa) — the Congo, a long-time Belgian colony, declared independence.

But a month later, the secessionist leader Moise Tshombe proclaimed independence in the province of Katanga. Europeans began to flee the region and floods of mercenaries arrived.

This is where alliances get muddled. The UN voted to send troops into the region to maintain law and order. Hammarskjöld was sympathetic to Congolese independence aspirations, as were many Western powers — at least officially. But powerful elements within the US, Belgium, and other European states backed the Katanga separatists: with the hope that Katanga might act as a buffer and halt the southward spread of African nationalism.

The mining behemoth Union Minière du Haut Katanga (now Umicore) also backed Katanga as it was anxious to keep hold of the region's vast uranium and cobalt deposits.

So when Hammarskjöld's plane came down, many around the world suspected foul play — even if they couldn't decide quite who was acting foully, and why.

The wildest of rumors emerged. One suggested that the plane had been attacked with help from the Romanian Embassy in Leopoldville, on the orders of the Russian KGB. Another claimed that Hammarskjöld survived the initial plane crash, but then was assassinated by European mercenaries.

Much of the UN panel report dismisses these claims as having "nil" probative value.

But the UN report addresses other theories with more cautious language: describing them as impossible to prove but also unwise to dismiss — notably, the claim that Hammarskjöld's plane was shot down by a Belgian mercenary known as the "Lone Ranger" who was committed to thwarting the UN's anti-colonial efforts. Belgium, after all, was at risk of losing its entire colonial prize.

Another theory — put forward in letters that were allegedly written by a clandestine South African mercenary agency, and released by the South African government in 1998 — purports that South Africa had carried out a determined operation to "remove" Hammarskjöld from office. The plot, known as Operation Celeste, allegedly had support from then CIA director Allen Dulles, who promised "full cooperation from his people," in addition to the Belgian Mining company Union Miniere. But the UN panel was unable to confirm the veracity of the documents: in part, because South Africa did not respond to the investigators' request for help.

The reinvestigation into Hammarskjöld's death did not pick up pace until 2011, when British historian Susan Williams published a book called Who Killed Hammarskjöld? This contended that the original inquiry into the plane crash, carried out by the Rhodesian government, amounted to a cover-up. And it presented new evidence on the case.

After the book was published, an independent commission of legal experts concluded that there was "convincing evidence" that the plane had been shot down.

In 2013, that commission chastised the US for withholding relevant documents from international investigators. The commission insisted that it was "highly likely" that the NSA and CIA had been monitoring local and regional radio traffic on the night that the plane went down.

But when the commission filed a Freedom of Information request for NSA intercepts, the request was denied.

Two former US government officials also accused Washington of withholding evidence. One of them, navy commander Charles Southall, was working at an NSA installation in Cyprus on the night of the crash and he said he overheard radio transmissions suggesting that Hammarskjöld's plane had been subjected to an aerial attack.

On the night of the crash, Southall testified, a senior officer contacted him "at about midnight" and asked him to return to the communications facility where he worked, to witness "something interesting."

The new UN panel also points a finger at US uncooperativeness. Notably, UN investigators have revealed that Hammarskjöld's Swiss-made CX-52 cipher encryption device may have been bugged by the NSA — via collusion with the device's Swiss manufacturers. This would have provided the US government with access to Hammarskjöld's travel arrangements on the night of his death.

None of this bodes well for the UN. After all, it cannot legally compel member states to hand over government files.

In fact, according to David Wardrop of the United Nations Association, a charity that analyzes UN activity, the UN has historically struggled with document sharing. For instance, Wardrop said, "there isn't a proper library of information at the UN" that gathers documents from prior peacekeeping missions — to shed light on lessons learned.

"Ban Ki-moon," argued Wardrop, "will have great difficulty in persuading states" to hand over Hammarskjold files, since nations generally resist intelligence sharing schemes.

But he will try. Early this month, Ban tasked UN Legal Counsel Miguel de Serpa Soares with pressing member states to reveal Hammarskjöld files.

Until they do, the idea that Hammarskjöld was murdered will be hard to put to rest — especially since the theory has, at times, been advocated at the highest government levels.

Soon after Hammarskjöld's crash, US President Harry Truman reportedly told the New York Times that the secretary-general had been "on the point of getting something done when they killed him. Notice that I said 'when they killed him.'"

This summer, representatives from the Harry S. Truman Presidential Library in Missouri told UN investigators that they have no documents that could explain Truman's comments.