A preliminary hearing for Army Staff Sergeant Robert Bales, accused of killing 16 Afghan villagers in a massacre last March 2012, was to enter its final stage, determining if he is to be tried for murder.
Army prosecutors and defence attorneys were expected to wrap up their arguments on Tuesday against Bales, 39, a father of two.
The hearing at a military base in Seattle is meant to determine whether there is enough evidence to warrant a court-martial.
Ultimately, an investigating officer will decide whether to recommend a court-martial, although a final determination will be made by Bales' brigade.
Military prosecutors are seeking the death penalty for Bales, accusing him of killing 16 people, mostly women and children, in two villages when he ventured out of his remote camp on two forays over a five-hour period in March.
Bales, a veteran of four combat tours in Iraq and Afghanistan, faces 16 counts of premeditated murder and six counts of attempted murder, as well as charges of assault and wrongfully possessing and using steroids and alcohol while deployed.
Worst civilian shooting in Afghanistan
The shootings in Afghanistan's Kandahar province marked the worst case of civilian slaughter blamed on an individual US soldier since the Vietnam War and damaged already strained US-Afghan relations.
Bales' lead defence attorney, John Henry Browne, has suggested that post-traumatic stress disorder or a concussion, combined with steroids and alcohol, may have played a part in the events of March 11.
In testimony that could undercut a defence that Bales was impaired, First Sergeant Vernon Bigham said over the weekend that Bales had undergone surgery for sleep apnea but did not complain of PTSD, traumatic brain injury or headaches.
Bigham, Bales' company supervisor, described the decorated serviceman as a capable sergeant "doing an outstanding job." He testified via video-link from Afghanistan's Kandahar Air Field.
Earlier in the hearing, witnesses testified that Bales had been upset by the lack of action over an attack on a patrol several days before the shootings in which one soldier had the lower part of a leg blown off by a bomb.
A 7-year old survivor of the killings also told the hearing how she hid behind her father when the gunman started firing. Her father died she said, cursing in pain and anger.
Such was the scale of the bloodshed that many of the villagers have found it hard to believe only one man was involved.
“They shot my brother in the head," Mullah Baran, brother of a shooting victim, said. "There was blood all over."
No evidence, however, has been presented to suggest there was more than one gunman.