In this 1959 file photo, Dag Hammarskjold, secretary general of the United Nations is pictured. (AP)

Around midnight on Sept. 18, 1961, a plane carrying U.N. Secretary General Dag Hammarskjold crashed nine miles from its intended destination, the town of Ndola in Northern Rhodesia, now the independent republic of Zambia. The 56-year-old Swede and 15 other people aboard the aircraft perished.

According to one account, Hammarskjold's body was found in the forest near the wreckage. He was "lying on his back, propped up against an ant hill, immaculately dressed as always, in neatly pressed trousers and a white shirt with cuff links." Hammarskjold is the only U.N. secretary general to have died while in office.

At the time, both the governments of Sweden and Northern Rhodesia claimed the incident was the result of pilot error. There was little evidence to be gleaned from the flight's sole survivor, an American sergeant who, before succumbing to his injuries, had said the plane experienced a series of explosions. A U.N. investigation the following year yielded no clear conclusion. It downplayed testimony from local villagers that a smaller, second aircraft may have shot down the plane.

Not surprisingly, the circumstances of Hammarskjold's death have always carried a suspicion of foul play. The Swedish diplomat was to meet with representatives from a breakaway state in the Congo -- a mineral-rich, fledgling nation that was coveted still by outside powers. There were many parties, even some in the United States, who perhaps did not want to see Hammarskjold's peace mission bear fruit.

On Monday, the U.N. General Assembly unanimously approved a motion asking current Secretary General Ban Ki-moon to appoint an independent panel of experts to investigate new evidence that has come to light regarding the 1961 plane crash.

Last year, a commission launched in the Hague issued a report, saying there was "persuasive evidence that the aircraft was subjected to some form of attack or threat as it circled to land." The Hammarskjold Commission concluded: "the possibility that the plane was in fact forced into its descent by some form of hostile action is supported by sufficient evidence to merit further inquiry."

The 2013 report had been spurred by the publication of a book in 2011 by British scholar Susan Williams. "Who Killed Hammarskjold?" suggested that the secretary general's plane had been shot down by mercenaries in the employ of the breakaway state of Katanga, which was then backed by Belgian mining interests. The book paints a picture of Western cover-up and collusion with the remaining white-supremacist settler states of Africa, all under the shadow of the Cold War.

Congo won its independence from Belgium in 1960, but almost immediately plunged into political crisis. Instability within the new nation's military prompted Belgium to send in troops and mercenaries and back the secession of Katanga province, a uranium, copper and cobalt-rich region where a powerful Belgian mining company had a considerable stake. Officials in Brussels did not want an independent Congo to nationalize the region's resources.

Frustrated by the U.N.'s inability to deal with the Belgian-backed forces, Congo's first prime minister, the charismatic socialist and African nationalist Patrice Lumumba, appealed to Moscow for aid. He was soon ousted by forces loyal to Congolese army chief Joseph Mobutu, whom, it later emerged, was the beneficiary of what was then one of the CIA's most lavishly funded support operations. Lumumba was murdered in 1961 while in the custody of Katangese troops.

Hammarskjold, who was no friend of Lumumba, still cared deeply about backing Africa's newly decolonized states. This was a time when the West looked warily at the emerging "third world" of independent nations, many of which were once subservient colonies but now, at least in the Western imagination, risked becoming Soviet proxies. African countries still ruled by white-supremacist governments supported Katanga's breakaway -- seeing it as a bulwark against African nationalism -- but Hammarskjold sought a unified Congo.

The flight he took to Ndola to meet with Katangese representatives left under the cover of darkness to avoid being tracked or intercepted by Katanga's air force. As the report of the Hammarskjold Commission details, there are still many questions over what happened aboard the flight and during its attempted descent. The crash site was sealed off and likely tampered with by North Rhodesian authorities before its discovery was even announced. Some investigators who have examined the case in recent decades are now convinced that the plane was shot down by a second aircraft, piloted possibly by a Belgian mercenary.

One of the most intriguing revelations discussed by the 2013 commission is the testimony of Charles Southall, an American official with the National Security Agency then stationed at a listening post in Cyprus. Southall, now retired, told the commission that a few hours before Hammarskjold's death, he received a communique from a supervisor telling him "something interesting is going to happen." Upon arriving at the U.S. facility, he heard over loudspeaker what seemed to be the voice of a mercenary announcing his attack on a transport plane.

Southall told the Wall Street Journal that the intercept was overheard through CIA, not NSA, circuit. "The CIA refused to confirm or deny the existence of any intercept following a Freedom of Information Act, or FOIA, request by The Journal. The agency upheld its decision after The Journal appealed it," the Journal reports.

Following the General Assembly's vote this week, though, the Obama administration may be obliged to declassify relevant documents.

For the Congolese people, Hammarskjold's death was a footnote to decades of war and misery. While Katanga ended its secession in 1963 (and its leader served briefly as Congolese prime minister), pro-Lumumba forces launched a rebellion. To quash it, white mercenaries and Western-backed forces rushed in. As an article in Foreign Affairs details, the United States, mostly through the CIA, spent tens of millions of dollars backing pro-Western figures in the Congo. It paid off massive bribes to other factions and even supplied "an instant airforce," piloted by Cuban exiles:

Washington was joining a particularly bloody conflict. When they seized rebel-held areas, the white mercenaries and government forces indiscriminately slaughtered the rebels and civilians they found there. Although there was no systematic counting of the casualties, it is estimated that at least 100,000 Congolese perished during this phase of the war.

By 1965, Mobutu, with American support, was in full control. He would go on to become one of the continent's most heinous tyrants, an "African Caligula." The legacy of his rule, and decades of fractious politics and weak governance, has left Congo a mess of warring militias and a society traumatized by mass violence.

Hammarskjold may not have prevented that, but a new investigation into his death will shine light on a very dark corner of the Cold War's history.