Staff Sgt. Robert Bales pleaded guilty in June in a deal to avoid the death penalty. On Thursday, he apologized for the first time for his "act of cowardice," but was unable to explain the atrocities of his March 11, 2012, attack.
"I'm truly, truly sorry to those people whose families got taken away," he said in a mostly steady voice. "I can't comprehend their loss. I think about it every time I look at my kids."
Bales, a father of two from Lake Tapps, Wash., was serving his fourth combat deployment when he left his outpost at Camp Belambay, in Kandahar province, in the middle of the night to attack two villages.
His attorneys previously made much of Bales' repeated deployments and suggested that post-traumatic stress disorder and traumatic brain injury may have played a role in the killings. They tried to paint a sympathetic picture of the soldier to contrast his own admissions and the testimony of angry Afghan villagers about the horror he wrought.
The massacre prompted such angry protests that the U.S. temporarily halted combat operations, and it was three weeks before Army investigators could reach the crime scene.
The Associated Press contributed to this report.