The UN has revealed that it received a flight recorder soon after the downing of a plane in 1994 which triggered the genocide in Rwanda.
The genocide began soon after the plane was downed
A newspaper earlier published details of a French police report into the missile attack on the plane.
The report concludes that the UN received the downed plane's recorder and blocked an inquiry into the crash.
The UN said there was no indication the recorder belonged to the downed plane, but would pass it to investigators.
Both Rwandan President Juvenal Habyarimana and Burundian President Cyprien Ntaryamira were killed in the attack.
UN chief spokesman Fred Eckhard told reporters that UN officials had found the flight recorder in a locked filing cabinet in the organisation's Air Safety Unit.
The "paper trail" which led to the discovery of the flight recorder, he said, indicated that UN officials at the time had apparently concluded that the flight recorder was not from the downed plane because it was in "pristine condition".
Mr Eckhard said an internal investigation would be conducted as to why this was not reported to senior peacekeeping officials at the time.
UN Secretary General Kofi Annan said on Wednesday that he was unaware the French investigation had been obstructed.
The French police report reproduced by Le Monde concludes that Rwanda's current President, Paul Kagame, gave direct orders for the rocket attack on Mr Habyarimana's plane.
Senior UN officials had "no knowledge" of the flight recorder
Mr Kagame was head of the mainly-Tutsi rebel Rwandan Patriotic Front (RPF) at the time.
The death of Mr Habyarimana, a Hutu, triggered the mass killings in which some 800,000 people died, most of them Tutsis.
Rwanda has rejected the French report, describing it as "fantasy".
A former RPF officer, Captain Josue Abdul Ruzibiza, has told the BBC that he was ordered to provide security as missiles were fired at the plane carrying the presidents of Rwanda and Burundi.
Capt Ruzibiza said two missiles had been brought from Parliament House, where his battalion was being housed, and were fired as the plane
carrying the two presidents came in to land.
The two men who actually fired the missiles, he added, are now senior officers in the Rwandan presidential guard and military intelligence.
Asked why he had kept quite so long, Captain Ruzibiza - now in exile - said he had lived in fear of his life.