Mr. Abram, now a lawyer and author, said in a telephone interview that he alluded to this version of events in a 2013 memoir, “Trona, Bloody Trona,” but has never shared his version of events with Mr. Southall. And as for official inquiries into Mr. Hammarskjold’s death, he said, “no one has ever contacted me.”

Ms. Williams, the British academic, wrote in her study of the crash that a “Belgian pilot called Beukels” claimed to have shot the plane down by accident as it was approaching Ndola, after trying to force the pilots to divert to another airstrip.

None of these accounts of the crash have been independently corroborated.

Mr. Hammarskjold was flying to Ndola from Léopoldville — now called Kinshasa — the capital of the Democratic Republic of Congo, which had gained independence from Belgium the year before. He was trying to broker an end to fighting in Katanga between United Nations troops sent to stabilize the country and secessionist forces backed by foreign mercenaries.

The wreckage of his plane was not spotted until the next afternoon, by a Royal Rhodesian Air Force plane based at Ndola. The sole survivor of the crash, Sgt. Harold Julien, an American security guard, died five days later without giving a coherent account of what happened.

Questions have been raised about the delay in locating the wreckage. The Royal Rhodesian officer who commanded the search, Squadron Leader John Mussell, wrote a detailed account a day after the crash, and recently made it available to The New York Times.

His chronology indicates that warplanes including a Canberra bomber began searching possible crash sites in the early morning without success. He then met at 2:23 p.m. with the American air attaché in Congo, Col. Benjamin Matlick, and other officials to plan a coordinated search. The wreckage was finally spotted at 3:10 p.m., 15 hours after the plane’s last radio message, and a second aircraft flew to the area “to help with positive identification,” Mr. Mussell’s chronology said.

In a note accompanying the document, Mr. Mussell, now 81 and retired in Wales, acknowledged that early searches were “directed to the wrong area,” but he rejected “any thought that we, in the Air Force, were not on our toes, and further, as conspiracy theories would hint, were in some way involved in the disaster.”