By Mark Doyle
BBC World Affairs correspondent
A former UN investigator has told the BBC his enquiries into the 1994 downing of the Rwandan president's plane were abruptly stopped against his wishes.
The downing of the plane sparked the 1994 genocide
At the time, he says he had obtained detailed allegations that the then rebel leader, and now President of Rwanda, Paul Kagame, was involved.
Michael Hourigan said his probe was stopped in 1997 and he was speaking now as he felt justice had not been served.
The shooting down of the plane sparked the genocide of up to a million people.
Radical ethnic Hutus reacted to the death in the crash of the Hutu president by trying to wipe out the minority ethnic Tutsis from which the rebels drew support.
No-one has been prosecuted for the downing of the plane.
'Doesn't make sense'
The shooting down of the Rwandan presidential jet, killing the Hutu president, in April 1994, continues to create international controversy.
Rwanda recently broke off diplomatic relations with France when a French judge accused President Kagame of involvement.
The former UN investigator who has now spoken to the BBC, Michael Hourigan, worked on several aspects of the genocide in 1996 and 1997.
He successfully prosecuted a number of Hutu leaders responsible for the mass killings, but also found witnesses who alleged that the Tutsi Paul Kagame, the current president of Rwanda, was involved in the plot to shoot down the plane when he was a rebel leader.
Mr Hourigan told the BBC from his home in Australia that senior UN officials instructed him to stop his enquiries.
"None of it makes sense," he said.
"That all of a sudden when we get the breakthrough and we start to actually get people coming forward saying: 'We were involved in the crash, you know, I fired a rocket which took the president's aircraft down' - when we're getting those people with that sort of quality information coming forwards and then we shut it down."
"I mean it didn't make sense to me then and it doesn't make sense to me now."
Senior UN officials say the enquiry into the plane crash was stopped because it was not within the mandate of the genocide tribunal. Mr Hourigan strongly disagrees.
Mr Kagame denies involvement in shooting down the plane
A spokesman for the then chief prosecutor of the tribunal, Louise Arbour, who is now the UN human rights commissioner, said she would not comment on Mr Hourigan's complaints because she had a duty of confidentiality.
Diplomats say Rwanda would almost certainly have stopped cooperating with the tribunal if its investigations targeted Mr Kagame.
The diplomats add that the foreign policies of western nations towards Rwanda are partly driven by guilt because the international community failed to stop the mass killings.
President Kagame has denied involvement in shooting down the plane, but adds that he does not regret the death of the former Rwandan leader, who he describes as a dictator.