The US has investigated a number of cases of alleged killings in Iraq
A US soldier has been jailed for eight months after admitting playing a part in the killings of four Iraqi men in April 2007.
Specialist Steven Ribordy, 25, admitted standing watch as fellow soldiers shot the four Iraqi prisoners, then threw their bodies into a canal.
He pleaded guilty to being an accessory to murder, in a deal that will see him testify against members of his unit.
Three soldiers have been charged with murder in connection with the case.
A total of seven men have been accused of involvement in the killings.
The unidentified Iraqis were captured by a US patrol after a shoot-out with insurgents near Baghdad, witnesses have told previous hearings of the US military court.
Prosecutors say the men were killed in retribution for casualties sustained by the soldiers' unit.
Ribordy told the military court that he stood guard as the men were shot, then later helped push one of the bodies into the canal.
He said he was sorry for his actions. "The reason I didn't say anything was because of loyalty to my comrades," he said.
Three men, Sgt John Hatley, Sgt 1st Class Joseph Mayo and Sgt Michael Leahy Jr, have been charged with premeditated murder, conspiracy to commit premeditated murder and obstruction of justice.
Last month another soldier was jailed for seven months under a similar plea deal to Ribordy's. Cases against two other soldiers are pending.
US troops in Iraq have faced several accusations of unlawfully killing civilians and abusing detainees, prompting inquiries into their conduct.
In the largest case, eight marines were originally charged in connection with the deaths of 24 Iraqi civilians in the town of Haditha, in Anbar province, on 19 November 2005.
Only one marine - Sgt Frank Wuterich - still faces charges in relation to the incident. He will be tried on nine counts of voluntary manslaughter later this year.