Dr Conor Cruise O'Brien has dismissed suggestions that US, British and South African agents were implicated in the death of the then UN secretary-general, Mr Dag Hammarskjold, in Africa in 1961.
Dr O'Brien was Mr Hammarskjold's representative in the breakaway Congolese province of Katanga at the time.
Mr Hammarskjold had been mediating between the Congolese government and Katanga in 1961, and was on his way to a meeting in Northern Rhodesia, now Zambia, when his plane exploded as it came in to land.
He died in the crash along with 15 others. Foul play was suspected, but an independent commission concluded that pilot error was probably to blame.
It emerged this week that letters presented to South Africa's Truth and Reconciliation Commission (TRC) describe meetings between the CIA, MI5 and the South African Institute for Marine Research, thought to be a front company. The letters are correspondence between two employees of this company, describe meetings with the intelligence organisations and suggest a plot to kill Mr Hammarskjold.
According to the chairman of the TRC, Archbishop Desmond Tutu, the authenticity of the letters has not been established, but they have been released as part of the commission's commitment to transparency.
"I have read an AFP report on this," Dr O'Brien told The Irish Times yesterday. "It clearly comes from people who don't understand what was happening at all. It suggests that Dag Hammarskjold was complicit in terrorism, which is rubbish. This is quite fantastic stuff. It would be worth journalists going to South Africa to see what they make of it.
"I think myself that he met his death as a result of foul play at the hands of French fascists. The OAS [Organisation de l'Armee de la Securit e] had sworn to get him," Dr O'Brien said.
"There were some pretty far-out people in the CIA who had an interest in discrediting Dag Hammarskjold. But they had no interest in his death at all. He was going to Rhodesia. When he was going to capitulate, why murder him?
"They had every interest in hushing up his death because they felt they would be blamed. They left without searching for the plane, leaving the survivors dying."