UPDATE: A 15-year-old Harriman boy faces a juvenile petition accusing him of second-degree murder in the shooting death of a Fort Campbell soldier.

Authorities filed the delinquency petition in Roane County Juvenile Court.

The teen defendant appeared in court Monday.

By law, prosecutors can ask the court to try the teen as an adult -- after a hearing.

It's not clear if that will happen.

PREVIOUS STORY: A teenager shot and killed a Fort Campbell soldier Sunday evening in Harriman, according to District Attorney General Russell Johnson.

Johnson said MarStratton Gordon, 23, was between deployments and on leave visiting his girlfriend at a home on Byrd Avenue when the shooting happened.

Investigators believed a domestic altercation led to a 15-year-old firing a single shot about 7:30 p.m. Sunday from a handgun at Gordon, fatally wounding him. The boy's name has not been released.

Gordon had attended Heritage High School and studied at Roane State Community College, according to his Facebook page. He was dating Amber Kinney, according to his Facebook page.

Gordon's half sister Alexis Terry spoke Monday night with 10News.

Terry, of DeKalb County, Ga., said her brother "Mar" was "a perfect gentleman," not someone prone to getting into fights, particularly with a teen male. She said the boy who shot Gordon was a sibling of Gordon's girlfriend.

She said Gordon had told family members the teen had several guns. Terry, however, said Gordon was not intimidated or troubled by his girlfriend's brother.

Terry said she understood he had dated the teen's sister for at least a year. She last spoke with Gordon about a month ago, she said.

Terry said she is eight months pregnant and that her half-brother was looking forward to being an uncle for her baby, a little girl.

She told 10News she didn't know details about the shooting.

"I know he didn't deserve to lose his life over an argument, and I don't know what a 15-year-old boy was doing with guns," Terry said.

Bonnie Carroll, a resident of Byrd Avenue, said the shooting came as a shock, and it surprised her the 15-year-old was involved because she always knew the teen to be "nice."

"I was shocked because to me it just doesn't seem like a thing that he would do," she said. "To do this, he had to have had a real good reason to do it. It just doesn't seem like a thing he would do."