The second-in-command of the Ugandan rebel movement, the Lord's Resistance Army, was executed by fellow officers, a diplomatic briefing has claimed.
Mr Otti is wanted by the International Criminal Court
The confidential document, shown to the BBC on condition of anonymity, says Vincent Otti was shot dead on 2 October at the home of LRA leader Joseph Kony.
The LRA has previously insisted Mr Otti was alive and under house arrest.
Mr Otti is wanted by the International Criminal Court for war crimes arising from the LRA's 20-year insurgency.
The briefing, based on witness testimony, says Mr Otti arrived at Mr Kony's home only to find it surrounded by the leader's personal guards.
Sensing that something was wrong, Mr Otti phoned his boss, but was reassured that all was well.
When he went inside the house, Mr Otti was met by a group of senior commanders.
Mr Kony arrived shortly after Mr Otti's death, the document says
One of the commanders stood up and pointed a pistol at Mr Otti, saying he was now under arrest, the briefing says.
The others followed his lead and Mr Otti was stripped of his shirt, bound and blindfolded. He reportedly began crying, asking what he had done wrong.
Mr Otti was led from the house, and was shot while begging for his life.
Later Mr Kony appeared, and told the assembled rebel officers that he would not tolerate indiscipline, it says.
The rebel leader apparently told his followers Mr Otti had been attempting to kill him, after receiving foreign funds.
Rumours of Vincent Otti's death have been circulating since October, but have been repeatedly denied by spokesmen for the LRA.
In early November a northern Ugandan politician and peace negotiator, Norbert Mao, told the BBC that he had spoken to Mr Kony, who denied killing Mr Otti.
He said Mr Otti was under arrest, accused of being a government spy.
Mr Kony and Mr Otti are two of four LRA commanders wanted by the International Criminal Court (ICC) for war crimes committed during their 20-year insurgency.
Mr Kony is in hiding in the remote north-east of the Democratic Republic of Congo.
The LRA were notorious for mutilating victims and kidnapping children to be fighters, porters and sex slaves but peace talks over the last year in southern Sudan have raised hopes that the conflict may be over.
But this week, President Yoweri Museveni said the LRA must sign a peace deal by 31 January or the war would be resumed.