A 57-year-old quest to uncover the cause of the plane crash that killed Dag Hammarskjold, the secretary general of the United Nations, and turned into the organization’s most haunting mystery, received new life on Tuesday, as the current secretary general extended the inquiry.

Secretary General António Guterres said he had reauthorized the work of a prominent jurist, Mohamed Chande Othman, who has been investigating whether previously untapped sources of information, including declassified intelligence archives, could yield clues into the 1961 crash of the aircraft carrying Mr. Hammarskjold and 15 others on an African peace mission.

Judge Othman, a former chief justice of Tanzania, produced a report for Mr. Guterres, released last October, that gave weight to a longstanding suspicion that Mr. Hammarskjold may have been assassinated.