( 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 . CNN 's Dana Ford and Matt Smith contributed to this report .
Army Staff Sgt . Robert Bales reportedly says he `` hid behind a mask of bravado '' He has pleaded guilty to more than 30 criminal charges , including 16 murder counts . A jury will decide whether the 39-year-old Bales will have a chance at parole .