LONDON — A prominent jurist conducting an investigation into the plane crash that killed United Nations Secretary General Dag Hammarskjold 57 years ago has made some progress in a longstanding quest for access to secret intelligence archives, a group following the inquiry said Friday.

In addition the jurist, Mohamed Chande Othman, a former chief justice of Tanzania, may widen the net to other countries and may seek clues in corporate archives from the era, said the group, the United Nations Association Westminster Branch.

Mr. Othman was reauthorized in March by the current secretary general, António Guterres, to establish whether the plane crash near Ndola, Northern Rhodesia — now Zambia — was caused by “an external attack or threat,” in the words of an earlier report.

At the time, Mr. Hammarskjold, an iconic Swedish diplomat, was traveling with 15 other people on a chartered DC-6 airplane on a mission to resolve a crisis caused by the secession of the southern province of Katanga in newly independent Congo. The question of how he died — initially ascribed by some investigators to pilot error — has proved to be among the most abiding mysteries in United Nations history.