(CNN) -- The sentencing phase begins Monday for Army Staff Sgt. Robert Bales who has admitted to gunning down 16 civilians in a 2012 rampage through two villages near his outpost in southern Afghanistan.
Bales pleaded guilty in June to more than 30 criminal charges, including 16 premeditated murder counts, in a hearing before a military judge.
The guilty plea spares the 39-year-old Bales the prospect of a death sentence in the killings. He now faces life in prison, but a jury of four officers and two enlisted personnel will decide whether he will have a chance at parole.
Bales admitted to slipping away from his outpost in southern Afghanistan and going on a house-to-house killing spree in two nearby villages in March 2012, a massacre that further strained ties between American troops and their Afghan allies.
But he has not offered an explanation for his actions.
In addition to the murder counts, Bales pleaded guilty to six counts of attempted murder, seven of assault and the use of illicit steroids and alcohol, but pleaded not guilty to a charge of obstruction of justice.
Bales is a member of the Army's 3rd Stryker Brigade Combat Team, an element of the 2nd Infantry Division. His attorneys have said the service made a mistake in assigning Bales to another combat tour despite evidence of post-traumatic stress disorder and a traumatic brain injury suffered during a combat tour in Iraq.
CNN's Barbara Starr and Devon Sayers contributed to this report.