A prominent jurist investigating the mysterious 1961 plane crash that killed the United Nations secretary general, Dag Hammarskjold, in southern Africa has concluded that the aircraft may have been attacked, and that four nations — Britain, Russia, South Africa and the United States — may be withholding information that could solve the puzzle.

The jurist, Mohamed Chande Othman, a former chief justice of Tanzania, issued his conclusions in a 95-page report posted Monday on the website of the United Nations, which retained him nearly three years ago to help sift through new evidence and a range of sinister theories that have proliferated in the decades since the crash.

Mr. Hammarskjold, a 56-year-old Swedish diplomat considered one of the most successful leaders of the United Nations, was on a mission to help settle a secessionist war in newly independent Congo, a former Belgian colony. His chartered aircraft, a DC-6, went down after midnight on Sept. 18, 1961, moments before its scheduled landing in Ndola, a town in what was then the British protectorate of Northern Rhodesia and is now Zambia.

Fifteen people aboard, including Mr. Hammarskjold, members of his staff and crew, were killed in the crash. The sole survivor, an American security officer named Harold Julien, died of injuries six days later.

The crash is one of the most enduring mysteries in the history of the United Nations, where Mr. Hammarskjold has been exalted as a model international statesman. He is the only person to have been posthumously awarded the Nobel Peace Prize. His name adorns buildings and plazas around the international organization’s New York headquarters.