A former ally of Rwandan President Paul Kagame has accused him of complicity in the death of a former president which sparked the 1994 genocide.
Theogene Rudasingwa said he heard Mr Kagame boast in 1994 that he ordered the shooting down of the plane carrying President Juvenal Habyarimana.
"By committing that kind of crime Kagame has the responsibility in the crime of genocide," he told the BBC.
President Kagame has repeatedly denied any involvement in the attack.
Mr Rudasingwa, who lives in the US, has fallen out with Mr Kagame in recent years and was sentenced in absentia in March to a 24-year jail term for threatening state security and propagating ethnic divisions.
Some 800,000 Tutsis and moderate Hutus died in the genocide which began on the evening of 6 April 1994, after Mr Habyarimana and Burundi's leader died in the plane crash.
Hutu militias then began a campaign of orchestrated killing against Tutsis.
One hundred days later, the Tutsi-led Rwandan Patriot Front rebel movement, led by Mr Kagame, captured Rwanda's capital, Kigali, prompting thousands of Hutus, including some of the killers, to flee into Democratic Republic of Congo.
In 2006, a French judge accused Mr Kagame and his allies of killing Mr Habyarimana - an allegation he dismissed as "ridiculous", insisting that extremist Hutus shot down the plane and blamed the RPF to provide a pretext for carrying out the premeditated slaughter.
He told the BBC's HardTalk programme in 2007: "I am not responsible for Habyarimana's death and I don't care, I wasn't responsible for his security and he wasn't responsible for mine either. He wouldn't have cared if I had died and I don't care that it happened to him."
But Mr Rudasingwa, the RPF's secretary general and a major at the time of the genocide, said in a statement released over the weekend on his Facebook page that despite public denials Mr Kagame was responsible.
He said the RPF leader was aware at the time of the implications of downing the plane.
"He has fully understood that an action like that one might trigger consequences which, as we know, in our country and the Great Lakes region actually produced that crime of genocide," Mr Rudasingwa told the BBC's Great Lakes Service.
'Lied for too long'
Mr Rudasingwa said he regretted that afterwards as Rwandan ambassador to the US he had promoted Mr Kagame's version of events.
"It is regrettable that I should have been one of the people who was instrumental in explaining and selling this version of the story about the killing of President Habyarimana, the president of Burundi and all the people who perished with them," he said.
Mr Rudasingwa, with other former RPF members, last year founded the Rwanda National Council, an organisation launched in exile in opposition to President Kagame's government.
He said he had waited a long time before deciding to talk about what really happened.
"I think the most important thing is that finally I have come out with the truth," he said.
"As to how long it has taken, it is two decades ago, but I think right now let's focus on seeing how this is the truth. But, yes, I've lied for too long."