Formal charges against Army Staff Sgt. Robert Bales, 38, are expected to be filed within a week. The 10-year Army veteran is being held in his own cell at Fort Leavenworth in Kansas.
Lawyer John Henry Browne said he and Bales had a lengthy and emotional talk.
"What's going on on the ground in Afghanistan, you read about it. I read about it. But it's totally different when you hear about it from somebody who's been there," Browne told The Associated Press. "It's just really emotional."
Military officials said Bales crept away March 11 to two villages overnight, shooting his 16 victims and setting many of them on fire. Nine of the victims killed were children.
U.S. officials announced that the soldier's trial will be held in the United States, and victims and witnesses will be allowed to testify. Bales could face the death penalty, something the military hasn't carried out since 1961.
Brown said Bales had a sketchy memory of the night of the massacre that has sparked protests in Afghanistan, endangered relations between the two countries and threatened to upend American policy over the decade-old war. He said the soldier remembers some details from before and after the killings, but very little during the time the military believes he went on a rampage.
Court records and interviews show that Bales had commendations for good conduct after four tours in Iraq and Afghanistan. But he also faced a number of troubles in recent years: A Florida investment job went sour, his Seattle-area home was condemned as he struggled to make payments on another, and he failed to get a recent promotion.
Legal troubles included charges that he assaulted a girlfriend and, in a hit-and-run accident, ran bleeding in military clothes into the woods, according to court records. He told police he fell asleep at the wheel and paid a fine to get the charges dismissed.
In March 1998, Bales was given a $65 citation for possessing alcohol at Daytona Beach, Fla. He did not pay the fine nor did he defend himself in court. A warrant was issued for his arrest, but it later expired.
Bales' wife, Karilyn Bales, issued her first statement, saying their family is struggling to make sense of what he's accused of, something that she says is "completely out of character of the man I know and admire."
The Associated Press contributed to this report.