Hammarskjöld's mission was delicate and dangerous. The Congo Crisis had pitted the central government in Léopoldville against the breakaway province of Katanga, whose secession was backed by Belgian mining interests and mercenary pilots. The UN had deployed peacekeepers, but the situation was spiraling. Hammarskjöld chose to fly to Ndola to meet Tshombe personally, a decision that required navigating not only political minefields but literal ones — the Katangese Air Force, small as it was, had Fouga Magister jet trainers capable of offensive sorties. The DC-6B was flown by Swedish captain Per Hallonquist, 35, first officer Lars Litton, 29, and flight engineer Nils Göran Wilhelmsson. It had been in service since 1952, powered by four Pratt & Whitney R-2800 radial engines. The route from Léopoldville to Ndola should have been straightforward.

Witnesses near Ndola reported a bright flash in the sky around one o'clock in the morning. The plane came down in dense bush, leaving a trail of wreckage through the trees. What followed compounded the tragedy. Three separate delays violated established search-and-rescue procedures: the first in raising the alarm that a plane might be in trouble, the second in confirming that it had not landed at any surrounding airport, the third in launching the search itself. The wreckage was not found until later that day. Sergeant Harold Julien, a UN security officer, was pulled alive from the debris and told hospital staff of a series of explosions before the crash. He died five days later. One medical report suggested Hammarskjöld died on impact; another indicated he might have survived had rescue come sooner. Norwegian Major General Bjørn Egge, the first UN officer to see the body, later described grass and leaves in Hammarskjöld's hands, suggesting the Secretary-General had crawled from the wreckage before dying.

Three official investigations followed — a Rhodesian Board of Investigation, a Rhodesian Commission of Inquiry, and a UN Commission of Investigation. None established a definitive cause. The Rhodesian board noted that two Swedish bodyguards had bullet wounds but attributed them to ammunition exploding in the fire. Eyewitness accounts of the sky flash were dismissed. The UN's own 2015 report, ninety-nine pages long, assigned "moderate" value to nine new witness accounts and transcripts of radio transmissions suggesting the plane was already on fire as it approached and that other jet aircraft were in the area. As recently as October 2024, the UN reported that some member states had released new information but that "specific and crucial" evidence continued to be withheld by a handful of governments. After more than sixty years, the file remains open.

The day after the crash, former US President Harry Truman said bluntly: "He was on the point of getting something done when they killed him. Notice that I said 'when they killed him.'" In 1998, Archbishop Desmond Tutu's Truth and Reconciliation Commission in South Africa uncovered letters implicating MI5, the CIA, and South African intelligence, including a claim that a bomb had been placed in the aircraft's wheel bay. Swedish aid worker Göran Björkdahl, interviewing witnesses in the 2000s, gathered testimony that the plane had been downed by another aircraft, possibly for the benefit of mining interests like Union Minière. In 2014, The Guardian published an account from former NSA employee Charles Southall, who recalled hearing an intercepted pilot's commentary: "I see a transport plane coming low. All the lights are on. I'm going down to make a run on it." None of these allegations has been officially confirmed. None has been definitively refuted.

The Dag Hammarskjöld Crash Site Memorial in the bush outside Ndola is under consideration for UNESCO World Heritage status. Hammarskjöld is buried in Uppsala, Sweden. In the Republic of the Congo, September 19, 1961, was declared a day of national mourning; the prime minister's statement condemned "the shameless intrigues of the great financial Powers of the West." Hammarskjöld was posthumously awarded the Nobel Peace Prize in 1961, the only person ever to receive the Nobel Peace Prize after death. His loss triggered a succession crisis at the United Nations that reshaped how the institution chose its leaders. The crash has been examined in books, documentaries, and films — but the central question endures. Whether Hammarskjöld's plane went down because of pilot error, mechanical failure, or deliberate attack, sixteen people died in the darkness near Ndola, and the world lost one of its most consequential diplomats at a moment when his presence might have changed the course of a war.