US media have reported that the group killed four handcuffed and blindfolded Iraqi prisoners with pistol shots to the head beside a Baghdad canal.
The men were apparently Shia fighters linked to the Mahdi Army, which controlled the West Rashid area of southwest Baghdad, according to the New York Times.
The killings were allegedly in retribution for an attack against the servicemen's unit.
In October, another US soldier, Steven Ribordy, was sentenced to eight months in prison for his role in the killings as part of a plea deal.
He initially denied a direct role in the deaths that occurred in the spring of 2007 but pleaded guilty to accessory after the fact.