The Marine, John Jodka, earlier pleaded guilty to charges of assault and conspiracy to obstruct justice in connection with the murder of 52-year-old Hashim Ibrahim Awad in Hamandiyah in April.
Under his plea deal with prosecutors, Jodka is expected to testify against other soldiers accused in the case.
In an unrelated case, a U.S. soldier has pleaded guilty to raping a 14-year-old Iraqi girl and helping to murder her and her family.
Specialist James Barker agreed to plead guilty in order to avoid a possible death sentence at his court-martial in the U.S. state of Kentucky, the soldier's lawyer said.
Barker is one of four current U.S. soldiers and one former soldier who have been charged in connection with the rape and murders last March in the Iraqi town of Al-Mahmudiyah.
(compiled from agency reports)