Shortly after midnight on September 18, 1961, a chartered DC-6 airplane carrying United Nations Secretary-General Dag Hammarskjold on a peacekeeping mission to the newly independent African nation of the Congo crashed in a forest near Ndola, in the British protectorate of Northern Rhodesia (now Zambia).

Hammarskjold and 14 other people aboard, including U.N. staffers and the plane’s crew, were killed; a single survivor died of his injuries six days later. Though inquiries by colonial authorities in Africa indicated the crash had been the result of pilot error, rumors of foul play surfaced immediately—and they have not stopped swirling since.

Today, Hammarskjold’s name is emblazoned on several buildings at U.N. headquarters in New York, while his death remains the biggest enigma in the organization’s eventful history. In 2017, the UN commissioned a new investigation of the crash, while the 2019 documentary Cold Case Hammarskjöld explores the long-running theory that Belgian or South African mercenaries may have shot down Hammarskjold’s plane to stop his diplomatic activities in the Congo, possibly even with the backing of U.S. and British intelligence.

The son of a former prime minister of Sweden, Hammarskjold started young in public service, working at the Ministry of Finance, the Bank of Sweden and later the Ministry of Foreign Affairs. He first became a Swedish delegate to the U.N. General Assembly in 1949, and in 1953 was elected as the second secretary-general, succeeding Trygve Lie of Norway. He was reelected to a second five-year term in 1957.

Hammarskjold was known for his hands-on approach to diplomacy, and his role in shaping the U.N. into an active force in making and keeping peace around the world. In 1954-55, he personally negotiated the release of 15 U.S. soldiers that China, which was not part of the U.N. at the time, had captured during the Korean War. He also helped assuage conflicts in the Middle East, including the Suez Crisis of 1956 and the clash between Lebanon and Jordan in 1958.

In mid-1960, Hammarskjold’s attention turned toward central Africa, where the Belgian Congo had recently become the independent Republic of the Congo (now Democratic Republic of the Congo). Shortly after independence was declared, the mineral-rich southern province of Katanga seceded, sparking a violent conflict that would pit U.N. peacekeeping troops supporting the republic’s new central government against Katanga’s separatist forces. The separatists, in turn, were backed by Belgian mining companies seeking control of Katanga’s resources (including uranium).

When Patrice Lumumba, the republic’s first democratically elected prime minister, appealed to the Soviet Union for military support, the Congo crisis also became a proxy battle in the Cold War. Lumumba was kicked out of office and assassinated (with the help of Belgium and the CIA) in early 1961. Shortly before his death, Hammarskjold was in Leopoldville (now Kinshasa) meeting with his successor, Cyrille Adoula, when he learned that U.N. forces had launched an aggressive attempt to expel foreign mercenaries from Katanga.

The U.N. attack, codenamed Operation Morthor, angered both U.S. and British authorities, who had not been consulted beforehand, as well as the mining interests backing the Katangese rebels. Hammarskjold arranged a meeting with Moise Tshombe, leader of the separatists in Katanga, to negotiate a cease-fire.

Hammarskjold’s plane, a chartered DC-6 aircraft known as the Albertina, was nearing the destination arranged for the secret meeting with Tshombe when it crashed into the forest early on September 18.

Two investigations into the crash by the British-run Central African Federation, which included Northern Rhodesia, found that pilot error was the likely cause, as the plane had been flying too low when it made its approach to the airport. But an official U.N. inquiry delivered an open verdict in April 1962, stating that it could not rule out sabotage or attack.

Questions surrounded the crash from the first. For one thing, writes Susan Williams in her 2011 book Who Killed Hammarskjold?, the British high commissioner at Ndola, Lord Alport, showed little concern after the U.N. plane failed to land at its scheduled time, instead insisting that Hammarskjold had decided to go elsewhere. Then there was the fact that search for the plane’s wreckage and crash site didn’t begin for hours after the crash, though witnesses had reported seeing a great flash in the sky soon after midnight.

Local residents in the area had seen a second plane in the sky that night, but their testimony was discounted or ignored by colonial authorities, the Guardian reported in 2011. The crash’s sole survivor, U.N. security officer Harold Julien, also spoke before he died of an explosion on board the plane, but the authorities assumed he was too ill and sedated to be taken seriously.

Two days after Hammarskjold's death, former U.S. President Harry Truman insinuated to reporters that the U.N. leader had been assassinated, saying he “was on the point of getting something done when they killed him. Notice that I said ‘when they killed him.’”

Such uncertainties have fueled several long-running conspiracy theories, centered around the powerful groups inside and outside Africa who hadn’t wanted Hammarskjold’s peacekeeping efforts in Congo to succeed.

According to one popular theory, Katangese separatists ordered a Belgian mercenary pilot, Jan van Risseghem, to shoot down the secretary-general’s plane. Van Risseghem was mentioned as a possible suspect in a cable sent by the U.S. ambassador to Congo just hours after the crash (but not declassified until 2014). But he was never interviewed by authorities about the crash; apparently flight logs gave him an alibi by showing he had not been flying at the time, and there are questions about whether he was even in the region.

Another long-standing theory centers on documents released from apartheid-era South Africa in the late 1990s, which suggest that a white militia group called the South African Institute for Maritime Research (SAIMR) orchestrated the plane crash that killed Hammarskjold—with the support of both British intelligence and the CIA. Though British officials claimed that the documents were likely Soviet forgeries, the theory has persisted.

Both theories are addressed in the 2019 documentary, Cold Case Hammarskjold. The film contains interviews that suggest the flight logs were forged and that van Risseghem (who died in 2007) admitted his involvement in the crash to a friend named Pierre Coppens four years later. A former SAIMR member recalls the group’s claims of successfully taking Hammarskjold down.

After former Secretary-General Ban-ki Moon took the lead in calling for renewed investigations, the U.N. appointed Mohamed Chande Othman, a Tanzanian judge, to review the crash in 2017. Othman didn’t reach a definitive conclusion, but later that year he reported that “it appears plausible that an external attack or threat may have been a cause of the crash, whether by way of a direct attack…or by causing a momentary distraction of the pilots.”

Othman’s inquiry was relaunched under U.N. Secretary-General António Guterres in 2018. Amid calls for countries around the world to be transparent and cooperate with his investigations, the New York Times reported that the Swedish government blocked a researcher’s request for access to related official documents on national security grounds—suggesting that even in Hammarskjold’s native land, there is still much that remains hidden in the long-running mystery of his death.