LONDON — It has been one of Africa’s enduring mysteries, redolent of days when mercenaries roamed the bushlands and outsiders scrambled to exploit a continent’s riches as it struggled for independence: How and why did Dag Hammarskjold, the United Nations secretary general, die?

The DC-6 airplane carrying Mr. Hammarskjold (pronounced HAH-mahr-shuhld), a Swedish diplomat, crashed in September 1961 as it approached Ndola, a mining town in Zambia, which at the time was called Northern Rhodesia. Official inquiries failed to explain what happened.

But last year, a United Nations panel concluded that there was “persuasive evidence that the aircraft was subjected to some form of attack or threat as it circled to land at Ndola.”

On Monday, Sweden formally asked the United Nations General Assembly to reopen the investigation. Significantly, the request included an appeal for all member states to release any hitherto unpublished records — a reference aimed largely at securing the declassification of American and British files, particularly intercepts thought to have been made at the time by the National Security Agency of the United States.