Raymond Williams had just retired and was looking forward to traveling out west with his wife and spending time with his three grandchildren. But all those plans were shattered on April 6, 2009. As Williams, 64, went to get the mail on that spring day, he was gunned down by a man he'd never met.

His wife found his body.

"She said, you know 'Matt! Matt! Somebody shot Dad,'" recalled Williams' son, Matt. "It didn't register. I'm thinking, 'OK where is he now? Did they take him to the hospital? What hospital is he in?' And before I could even get another word out, she goes 'And he's dead.'"

A short time earlier, the same gunman had killed a teenager and wounded a woman at a store in the same working-class town of Altoona in central Pennsylvania.

The gunman, Nicholas Horner, was a husband, a father, and a veteran soldier who had been awarded multiple medals for his service in Iraq, including a combat action badge. Less than a year after returning from combat, Horner faced two first degree murder charges and the possibility of the death penalty.

"Not in a million years could I believe this was true because Nick would never, he could never hurt anyone," said Horner's mother, Karen. "I know Nick. Nick pulled the trigger, but that wasn't Nick."

After he returned from Iraq, Horner was a different person, his mother said. He barely left his home and, oftentimes, his wife would find him crying in the corner of the basement.

"He wasn't my little boy anymore," said Karen Horner. "You could see in his eyes, he had seen things and done things that probably none of us should ever see."

There was no question that Horner had committed the crime, and his attorney would not argue otherwise. The question was whether Horner was to blame.

"I argued to the jury in my opening (statement), I said I believe that the Iraq war came home that day," said defense attorney Tom Dickey.

Horner was one of the thousands of soldiers and veterans diagnosed with post-traumatic stress disorder after returning from fighting in Iraq or Afghanistan. In total the military says 90,000 currently serving troops who fought in those wars were diagnosed with PTSD. The number from the Veterans Administration is well over 200,000.

After a decade of combat, PTSD is being used as a criminal defense in the courtroom. Horner's case would test whether this combat-related illness could be accepted as a defense for murder.

The crime

The morning of the shootings, Horner dropped his kids off at school and went shopping with his wife. An argument ensued and he stormed away, armed with his .45-caliber handgun.

"He left Walmart and went to a bowling alley, where he sat all afternoon and drank several ... at least two pitchers of beer," said prosecutor Jackie Bernard.

Horner ordered food, talked to people and then walked over to the Subway sandwich shop intending to rob it, the prosecutor said.

Witnesses said Horner pounded on the shop's back door trying to get in. Evidence showed that Horner cut the electrical wires to the restaurant and even tried to shoot out the utility box.

Horner's lawyer argues he was in the middle of a PTSD episode and, to him, the Subway looked like a building in Iraq.

"Why do you in broad daylight enter from the rear and announce yourself by firing, you know, five or six shots?" Dickey said.

Once inside, Horner shot and killed Subway worker Scott Garlick, a teenager two months shy of graduating high school. He then shot and injured another worker, Michelle Petty, and stole about $130.

"And when he left, he walked over to Scott's body as he lay bleeding there and said to him: 'Sorry, I didn't wanna have to do that to you,'" said Bernard.

Several blocks away, Horner spotted Raymond Williams. Prosecutors argue he killed Williams for his car keys to try to get away.

During the trial Horner pleaded "diminished capacity" in an effort to persuade the jury to find him not guilty because of PTSD.

PTSD on trial

The same year as the Altoona shootings, an Oregon jury found Iraq war veteran Jessie Bratcher guilty of murder but legally insane. So, instead of serving 25 years in a maximum security prison, Bratcher was provided treatment at the Oregon State Hospital. . A medical review board keeps track of his progress, and his attorney says he could be released as early as this month.