Statements by a US Army sergeant when he surrendered after a two-village killing spree in southern Afghanistan suggest he knew what he was doing on the night 16 civilians were massacred, prosecutors said.
The March 11 attack - one of the worst atrocities of the Iraq and Afghanistan wars - prompted the US to halt combat operations for days in the face of protests and military investigators could not reach the crime scenes for a month.
A medic told a hearing at Joint Base Lewis-McChord, Washington, that he saw Staff Sgt Robert Bales covered in blood after the attacks and knew from the pattern of the staining it was not his own. He asked where it came from and where he had been.
Bales shrugged, the medic, Sgt 1st Class James Stillwell, said. "If I tell you, you guys will have to testify against me," Sgt Stillwell quoted him as saying. That was one of many statements attributed to Bales that suggest he knew he was responsible, prosecutors say.
The remarks, offered by fellow soldiers giving evidence for the US government, could pose a high hurdle for defence lawyers who have indicated that Bales' mental health will be a big part of their case.
The evidence is part of a preliminary hearing being held to help determine whether the case goes to a court martial. The defence has noted that Bales, 39, was serving his fourth deployment and had suffered from post-traumatic stress disorder as well as a concussive head injury in Iraq.
Father-of-two Bales, from Lake Tapps, Washington, faces 16 counts of premeditated murder and six counts of attempted murder over the attack on the villages of Balandi and Alkozai, which counted nine children among its victims.
A prosecutor's opening statement and witness evidence on Monday suggested Bales spent the evening before the massacre at his remote outpost of Camp Belambay with two other soldiers, watching a film about revenge killings, sharing contraband whiskey from a plastic bottle and discussing an attack that cost one of their comrades a leg.
Within hours, a cape-wearing Bales slipped away from the post and embarked on a killing spree of his own, said prosecutor Lt Col Jay Morse. He attacked one village before returning to Belambay to wake a colleague and report what he had done but the colleague said he did not believe Bales and went back to sleep.
Bales headed out again, Lt Col Morse said, and attacked the second village, bringing his death toll to 16 before returning once again before dawn, bloody and incredulous that his comrades ordered him to surrender his weapons. His return to the base was captured on surveillance video, Lt Col Morse said.