A third US serviceman has pleaded guilty to charges related to the murder of an Iraqi civilian.
Lance Corporal Tyler Jackson was escorted into his court martial
Marine Lance Corporal Tyler Jackson, 23, told a military court that his unit deliberately targeted Hashim Ibrahim Awad, believing he was an insurgent.
Eight US personnel were initially accused of murdering Mr Awad.
But another marine and a navy serviceman have already pleaded guilty to lesser charges. Five others have yet to stand trial.
L/Cpl Jackson pleaded guilty to reduced charges of aggravated assault and conspiracy to obstruct justice, in return for his co-operation.
He is due to be sentenced on 16 November. He faces a maximum of 15 years in jail, although the sentence is expected to be reduced because of his plea deal.
The eight US personnel are accused of taking their 52-year-old disabled victim from his home in Hamdaniya, near Baghdad, and shooting him dead.
The accused are also alleged to have covered up the circumstances of his death.
In his testimony, L/Cpl Jackson said his squad had believed Mr Awad was responsible for planting roadside bombs, including one which killed four marines.
He said he had agreed with the plan to kill Mr Awad.
"Everyone fired rounds, including myself, but I fired my rounds above him. I knew he was going to be shot, but I didn't want to be the one to do so," L/Cpl Jackson said.
It was only later that they realised they had grabbed the wrong man, the court heard.
The accused were taken out of Iraq and held at Camp Pendleton in California after local Iraqis reportedly told marine leaders about the alleged incident.
The shooting of Mr Awad is one of several killings in Iraq in which US troops are accused of murdering Iraqi civilians.