More than 56 years after a plane crash killed Dag Hammarskjold, the secretary general of the United Nations, an authoritative report released on 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 longstanding suspicion that Mr. Hammarskjold may have been assassinated.

The crash, during the overnight 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.

Judge 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 Mr. Hammarskjold’s chartered DC-6 airplane to crash in what is now Zambia.