(CNN) — The Army sergeant who admitted to gunning down 16 civilians in a 2012 rampage through two villages near his outpost in southern Afghanistan reportedly apologized Thursday, describing the massacre as an "act of cowardice."
Army Staff Sgt. Robert Bales pleaded guilty in June to more than 30 criminal charges, including 16 premeditated murder counts.
The 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.
"I hid behind a mask of bravado," Bales said, according to Drew Mikkelsen of CNN Seattle affiliate KING, who was tweeting from court.
Bales reportedly told the jury he took steroids in Afghanistan. He also took pills to help him sleep and drank sporadically, Mikkelsen wrote.
When asked who is responsible for what he did, Bales reportedly said: "I am. I am responsible."
Bales admitted to slipping away from his outpost 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.
His sentencing hearing began this week at Joint Base Lewis-McChord, near Tacoma, Washington. Bales testified for about 40 minutes, according to Mikkelsen.
Bales reportedly said the killings were an "act of cowardice" and apologized to his victims.
"I am sorry ... I murdered their family," said Bales, according to Mikkelsen.
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. He 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.