After 56 years and many investigations, there is new hope that secrets lurking in Western intelligence archives could solve the biggest whodunit in United Nations history: the mysterious death of Secretary General Dag Hammarskjold.

Whether the keepers of those archives will allow access remains an open question.

Mr. Hammarskjold, an iconic Swedish diplomat who was the second secretary general of the world body, died with 15 others when their plane, a chartered DC-6, crashed just after midnight on Sept. 18, 1961, minutes from its destination: an airfield in Ndola, in what was then the British protectorate of Northern Rhodesia and is now Zambia.