The US has investigated a number of cases of alleged killings in Iraq
A US soldier has been jailed for seven months and given a dishonourable discharge for conspiracy to murder four Iraqi men in April 2007.
Specialist Belmor Ramos pleaded guilty to knowingly standing guard as the men were blindfolded, shot and their bodies dumped in a canal.
It was alleged that the killings were in retribution for casualties sustained by the soldiers' unit.
Three other soldiers were charged in connection with the case on Wednesday.
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.
A pre-trial hearing will be held at a later date to determine whether there is sufficient evidence for a court martial.
Ramos told the court that he had agreed with the decision to kill the prisoners and had heard the shots, but did not personally witness the killings.
The military judge said he would have sentenced Ramos to 40 years in prison had he not agreed to testify against the other soldiers alleged to have been involved in the killings.
A military spokesman told the BBC that the verdict would be subject to an automatic review, the military equivalent of an appeal.
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.