New evidence to be submitted to the United Nations' General Assembly this week could help shed light on one of the enduring mysteries of the 20th century — namely, was the 1961 death of the second UN Secretary General an accident or an act of murder?

Dag Hammarskjöld died in a plane crash in Ndola, Northern Rhodesia — now Zambia — along with 15 others on Sept. 18, 1961.

The 56-year old Swedish diplomat was in Africa to try to unite the Congo, but faced resistance from a number of multinationals, often supported by mercenaries and openly hostile to the UN, who coveted the area's mineral wealth.

The crash has been a source of widespread speculation for decades, which has ramped up thanks to evidence uncovered in the last few years.

Searchers walk through the scattered wreckage of the DC6B plane that had carried Dag Hammarskjöld, in a forest near Ndola, Zambia, Sept. 19, 1961. (Associated Press)

That includes testimony from a former U.S. National Security Agency intelligence officer who claims he heard a recording of another pilot attacking the plane, as well as a Belgian pilot who says that he accidentally shot the plane down after being hired to merely divert it.

UN spokesperson Stéphane Dujarric said a three-member panel appointed by current Secretary General Ban Ki-moon recently travelled to Zambia to interview new witnesses and gathered new documents from public and private archives in the United Kingdom, Sweden and Belgium.

Ban is examining the panel's report and will make his own recommendations on how to proceed before it is distributed to the General Assembly — expected to happen this coming week.

The evidence could result in a new UN probe into the crash, which would be the first since an inconclusive 1962 UN inquiry. But Dujarric says that decision will be left up to member states.

'They killed him'

Suspicions that the plane was shot down more than half a century ago are not new.

Just two days after the crash, former U.S. President Harry Truman told The New York Times, "Dag Hammarskjöld was on the point of getting something done when they killed him. Notice that I said, 'When they killed him,'" Truman emphasized, without elaborating.

At the time of the tragedy, Hammarskjöld was on a peace mission to unite the Congo, which had just gained independence from Belgium.

He was flying from the capital, Léopoldville (which later became Kinshasa), to meet with secessionist leader Moise Tshombé, who had declared the mineral-rich southeastern province of Katanga an independent state.

The men were to meet in Ndola, in the neighbouring British colony of Northern Rhodesia, because of ongoing fighting in Katanga.

But just after midnight on Sept. 18, Hammarskjöld's chartered DC6 crashed in a forested area about 14 kilometres from the Ndola airport. Hammarskjöld received a posthumous Nobel Peace Prize later that year.

Secretary General Dag Hammarskjöld, left, receives credentials from Canadian diplomat Charles Ritchie at the UN in New York in 1958. Canadian Alice Lalande, Secretary to the head of the UN mission in the Congo, was also killed in the 1961 crash. (Canadian Press)

Renewed speculation of foul play arose in 2011, on the 50th anniversary of his death, when Susan Williams published the book Who Killed Hammarskjold? The UN, the Cold War and White Supremacy in Africa.

It offered a new analysis of the evidence, including previously unseen documents, photographs, as well as testimony from eyewitnesses, many of them African, who had either not participated or not been taken seriously by Rhodesian officials or the UN.

Williams argues this was a direct result of the white minority regime in place in Northern Rhodesia at the time.

"[The locals'] testimony was dismissed, disqualified, ignored, in some cases changed," said Williams, adding that some people were afraid to come forward.

Williams said a number of local eyewitnesses told her that they saw a second, smaller aircraft "that dropped something that looked like fire" on top of the bigger plane right before it went down.

Queen Elizabeth addresses the General Assembly, as her husband Prince Philip, sits at left, Oct. 21, 1957. Hammarskjöld sits on the far left, behind the Queen. (John Rooney/Associated Press)

The crash killed the UN chief and 15 others, including Alice Lalande, Secretary to the head of the UN mission in the Congo, who was from Joliette, Que. She was the only Canadian and the only woman on board the aircraft.

Williams has two primary theories: that it was an assassination or a hijacking gone wrong.

The second theory is based on testimony from a former Belgian pilot known only as "Beukels," who claimed in 1967 that he accidentally downed Hammarskjöld's plane while trying to divert it with warning shots.

A group of international jurists known as the Hammarskjöld Commission sums up Beukels' testimony in a 2013 independent investigative report by stating he claimed he was acting on behalf of a group representing "a number of European political and business interests" who wanted to "persuade [Hammarskjöld] of the case for Katanga's continued independence."

No shortage of suspects

Henning Melber, former director of the Dag Hammarskjöld Foundation, explains the UN chief's mission to unite the Congo automatically pitted him against colonial settlers desperate to hold onto power and Katanga's vast mineral resources.

That included Belgians, French, the British and "mercenaries of all shapes and colours," said Melber, who also helped establish the Hammarskjöld Commission of Inquiry.

For the Rhodesian Federation, which included modern-day Zambia, Malawi and Zimbabwe, Katanga acted as a barrier against the southward migration of African nationalism. One year earlier, in 1960, the UN had admitted 17 new member states, 16 of which were newly independent nations in Africa — including the Congo.

While the European colonies were slowly dying, the U.S. and USSR were jockeying to expand their Cold War sphere of influence, as well as their share of the resources.

"One needs to remember that the uranium that was used in the bombs on Hiroshima and Nagasaki came from the Katanga province," said Melber.

On Feb. 15, 1961, Hammarskjöld attends a meeting at the UN headquarters in New York. That was seven months before he died in what is now Zambia, a death still shrouded in mystery. (Associated Press)

The commission said the "chaotic, contradictory, frequently inexplicable and in places irresponsible reaction (or lack of reaction) of the Federation authorities" following the crash has helped fuel the rumour mill.

For example, it took the Rhodesian RAF pilots 15 hours to "officially" locate the aircraft, while Daily Telegraph correspondent Ian Colvin said he had spotted the crash site — crawling with police — six hours earlier in a chartered Cessna.

Other oddities: Some of the bodies of the victims had bullet holes, and a playing card — rumoured to be the ace of spades — was found in Hammarskjöld's collar.

The sole survivor of the crash was American security officer Sgt. Harold Julien. In his testimony, he spoke of "sparks in the sky" and said the plane "blew up," but the lead inspector of the local investigation dismissed his statements as "rambling."

According to the Hammarskjöld Commission, hospital staff said that although he was badly burned, Julien was often coherent and lucid. He died six days after the crash.

'The Lone Ranger'

Some of the most compelling testimony of foul play comes from Charles Southall, who in 1961 was an intelligence officer stationed at the U.S. National Security Agency's naval communications base in Cyprus.

He said he heard a pilot shoot down Hammarskjöld's plane and that the CIA and/or the NSA have a recording of it.

"The watch supervisor called me and said, 'Come [into work] about midnight, something interesting is going to happen,'" Southall told CBC.

That's when he said he heard a recording of the crash that somebody told him was seven minutes old, which, according to Southall, "meant that somebody down there in the Ndola area, also waiting for this to happen, made a recording of it, put a date-time stamp on it and… sent it off."

In his statement to the Commission of Inquiry, Southall recalled the pilot saying, "I see a transport plane coming low. All the lights are on. I'm going down to make a run on it. Yes, it's the Transair DC6. It's the plane."

Then, the sound of cannon fire, at which point the voice, which he described as cool and professional, became animated: "I've hit it. There are flames. It's going down. It's crashing."

Southall, now 82, believes the voice he heard was that of a Belgian mercenary pilot nicknamed "The Lone Ranger."

Hours before the wreckage was officially located, the U.S. Ambassador in the Congo, Edmund Gullion, sent a cable to Washington speculating that the secretary general's plane might have been attacked by a known Belgian mercenary.

"There is possibility he was shot down by the single pilot who has harassed UN operations," Gullion wrote.

The document, which was released by the State Department following a Freedom of Information Act (FOIA) request on behalf of the Hammarskjöld Commission, identified the pilot as Vak Riesseghel, likely a misspelling of Katanga Air Force Commander Jan Van Risseghem.

A UN Military report stated that Van Risseghem, a former South African and Royal Rhodesian AF pilot, had been arrested and repatriated to Brussels 10 days before the crash, but had managed to return to Katanga.

"As long as he is still operating he may paralyze the air rescue operations," wrote Gullion in the 1961 cable.

The FOIA request, which asked for any recording, transcription or radio message intercepted the night of the crash, produced two additional documents.

But the Hammarskjöld Commission says that more than 50 years on, they remain "top secret" and sealed for national security reasons.