More than 56 years after a plane crash killed Dag Hammarskjold, the secretary-general of the United Nations, an authoritative report released Wednesday said it appeared plausible that an “external attack or threat” may have downed the airplane carrying him and 15 others on an epochal peace mission in Africa.

The finding by Judge Mohamed Chande Othman, a senior Tanzanian jurist who was asked by the United Nations to review both old and newly uncovered evidence, gave weight to a long-standing suspicion that Hammarskjold may have been assassinated.

The crash, during the overnight hours of Sept. 17-18, 1961, remains a painful open wound in the history of the United Nations and one of the 20th century’s most enduring mysteries.

Othman’s 63-page report offered a further rebuttal of the idea, advanced in inquiries soon after the crash, that pilot error or some other accident had caused Hammarskjold’s chartered DC-6 airplane to crash in what is now Zambia.

Moreover, Othman’s conclusion reinforced the theory that the plane had been deliberately brought down, either by what the judge called “direct attack” or a “distraction” that diverted “the pilots’ attention for a matter of seconds at the critical point at which they were on their descent.”

At the time, Hammarskjold was flying to Ndola, in what was then Northern Rhodesia, for negotiations to end secession and civil war in the neighboring mineral-rich Congolese province of Katanga. The Katangese separatists were supported by Western political and mining interests not eager to see Hammarskjold’s diplomacy succeed.

In recent years, much attention has focused on the extent to which Western governments and their intelligence agencies, including those of Britain, the United States and Belgium, the former colonial power in Congo, have withheld information relating to Hammarskjold’s death.

Othman said these countries had provided some “valuable new information” in response to his requests.

At the same time, he said, the “burden of proof” had now shifted to member states of the United Nations to “show that they have conducted a full review of records and archives in their custody or possession, including those that remain classified, for potentially relevant information.”

His remarks seemed to reinforce many earlier suggestions that, for whatever reason, Western governments were loath to disclose their full knowledge about what had befallen Hammarskjold, a Swedish diplomat killed at a tipping point in African history between colonial rule and independence.

At the time, Congo had achieved a fraught independence from Belgium, while British and Portuguese colonial rule still prevailed farther south. The secession of the southern Congolese province of Katanga illuminated the competition among rival superpowers and commercial interests for influence over Africa’s future.

For supporters of Katanga’s secession, Hammarskjold was a reviled figure.

Such were the concerns about his safety that his airplane, call-sign SE-BDY, flew a circuitous route, skirting Congolese territory and observing near-total radio silence before it approached Ndola.

Myriad theories about the causes of the crash have emerged, including pilot miscalculations of their altitude and the sudden appearance in the nighttime skies of a secessionist warplane flown by a mercenary pilot.

Othman’s report said: “There is a significant amount of evidence from eyewitnesses that they observed more than one aircraft in the air, that the other aircraft may have been a jet, that SE-BDY may have been on fire before it crashed and/or that SE-BDY was fired upon or otherwise actively engaged by another aircraft. In its totality, this evidence is not easily dismissed.”

Secretary-General António Guterres, who released Othman’s report, called its findings “insufficient to come to conclusions about the cause or causes of the crash.” But Guterres also said it seemed “likely that important additional information exists.”

Susan Williams, a British academic whose 2011 book “Who Killed Hammarskjold?” inspired the latest phase of high-level interest in the crash, said the report “reinforces my strong suspicion of foul play.”

“The onus is now on the U.K., the U.S., Belgium, France and South Africa, to release all relevant documents, including the secret records of their security and intelligence agencies and all intercepts” of radio traffic relating to the case, she said in an interview. She also urged multinational companies operating in the area to “release relevant records.”

Othman’s report evoked an era when rebellious forces, white mercenaries and U.N. soldiers battled in breakaway Katanga as foreign intelligence agents chronicled and perhaps steered events for governments back home. U.S. aircraft with high-powered radio transmitters flew clandestine intelligence missions, the report suggested, and U communications were routinely intercepted.

One issue centered on the capability of Katangese secessionist forces and their foreign hires to attack Hammarskjold’s plane.

At the time the secessionists were using French-built Fouga Magister warplanes. Earlier inquiries had discounted their deployment because they lacked flying range, despite witness testimony about a second plane seen that night as Hammarskjold’s DC-6 approached Ndola.

But more recent evidence suggested that one or more Fouga Magisters could have flown a combat mission or harassed the DC-6 at a critical moment on its approach.

Othman also said there had been evidence that the British colonial authorities had sought to ensure that early inquiries ascribed the crash to pilot error. But, he said, that conclusion should now be considered “logically unsound.”

He noted that, in the past few years, the United States had acknowledged the activities of CIA officers in the Congo region and changed the narrative about the presence of Fouga Magisters in Katanga and U.S. DC-3 Dakotas on the ground in Ndola at the time of the crash.

“Judging from history and the manner in which potential new information has emerged over the years,” his report said, “it is still likely that additional information will be located, unearthed or made available.”