“A military panel just returned findings of guilty to premeditated murder and conspiracy to commit murder against Sergeant (Michael) Leahy,” Lieutenant Colonel George Wright told AFP.
“The panel has moved to the sentencing phase,” he said of the military court in Vilseck, Bavaria.
Leahy, one of several soldiers charged in the killings, could be jailed for life.
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 Shiite fighters linked to the Mahdi Army militia, 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, Specialist 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 shooting deaths that occurred in the spring of 2007 but pleaded guilty to accessory after the fact.