Spc James Barker agreed to the plea deal at the start of his court-martial in the US to avoid the death penalty, his civilian lawyer said.
A criminal investigation began in June into the alleged killing of the family of four in their home in Mahmudiya, south of Baghdad, in March 2006.
Spc Barker is one of four US soldiers charged with premeditated murder.
They are alleged to have helped a former private - who has since left the army - to plan, carry out and cover up the attack. Two of the soldiers could face the death penalty if found guilty.
All four soldiers belong to the 2nd Brigade of the elite 101st Airborne Division.
Spc Barker agreed to co-operate with prosecutors and will testify against the others, his civilian lawyer David Sheldon said.
The others charged with rape and murder were Pfc Jesse Spielman, Sgt Paul Cortez, and Pfc Bryan Howard.
In addition, former soldier Steven Green has been charged in a civilian court and is awaiting trial in a Kentucky jail. He pleaded not guilty in July.
Earlier on Wednesday, Sgt Cortez was arraigned at the start of his court-martial.
Source :BBCGüncelleme Tarihi: 20 Eylül 2018, 18:16