LONDON — Last month, the outgoing secretary general of the United Nations, Ban Ki-moon, published a memo following up the inquiry he instigated last year into the death, nearly 55 years ago, of one of his predecessors in the role, the Swedish diplomat Dag Hammarskjold. Hammarskjold perished, along with his entourage, just after midnight on Sept. 18, 1961, when his chartered airliner crashed near Ndola, in what was then Northern Rhodesia, now Zambia.

More than half a century on, the cause of the crash remains unknown. Two recent commissions — the first of which I led, the second set up by the United Nations General Assembly in response to my commission’s findings — appear to have been stalled by a United States agency that may hold critical evidence pointing to the cause of the disaster.

“I would again urge all member states to continue their search for relevant documents and information, and to review for potential disclosure information which remains classified or undisclosed for other reasons,” Mr. Ban said in his statement.

Mr. Ban also released a new denial from the United States mission to the United Nations that any record exists of United States Air Force aircraft being present at Ndola on the night of the crash. Given that, as my commission reported, two American planes were visibly parked on the airport tarmac, and since Don Gaylor, then the United States air attaché in Pretoria, wrote in his memoirs that he had been ordered by the Pentagon to go meet Hammarskjold at Ndola, this new denial appears to signal a retreat from an initial acknowledgment to my commission by the National Security Agency that it held relevant intercept records.