LONDON — 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.

The three official inquiries that immediately followed suggested pilot error was the cause, but the third of the reports, by the United Nations Commission of Investigation in 1962, said sabotage could not be ruled out. That possibility helped feed suspicions and conspiracy theories that Mr. Hammarskjold, 56 at the time of his death, had been assassinated.

Since then, independent investigators and academics have spent years collecting and scrutinizing evidence that had been dismissed or suppressed, bolstering the theory of foul play. In her 2011 book “Who Killed Hammarskjold?,” Susan Williams, a University of London scholar of African decolonization, concluded that “whatever the details, his death was almost certainly the result of a sinister intervention.”