Norma Gamble was cooking dinner in her Mid-City home Sunday evening when she heard a gunshot from upstairs.

She ran to the second floor, where a teenage boy who had been living with the family and two teenage girls were hanging out. She found one of the girls on the floor, bleeding profusely from a bullet wound to her lower back.

“My stomach hurts,” said the 15-year-old. “Help me.”

Paramedics took the girl from the home in the 1800 block of Wellington Road to Cedars-Sinai Medical Center, where she was pronounced dead.

Los Angeles Police Department detectives Monday were still trying to piece together what happened. But detectives said the 16-year-old boy had been discussing Russian roulette and was loading and unloading a revolver as the girls watched.

Detectives said one of the girls became uneasy as the boy kept playing with the gun. The girl, identified by the L.A. County coroner’s office as Alexis Wallace, stood to leave the room. As she was walking out, the boy pointed the gun at her and pulled the trigger, not realizing that there was a bullet in the chamber, police said.

When Gamble came across the bloody scene, she said, the boy was distraught, running around the room in a “chaotic state.”

“The gun just went off,” he said in disbelief, throwing the weapon onto his bed.

“What do you mean? We don’t own any guns,” Gamble replied.

The boy had recently arrived at the two-story home on a tree-lined street in the historic Lafayette Square section of Mid-City. Gamble said her brother, who lives with her in the home, was a close family friend of the boy and his mother, who lives in Compton. He decided to take the boy in, hoping the move would keep him out of trouble.

The teenager’s father had been absent for years, the Gamble family said. The family was hoping to give the boy a strong set of role models, and a safer neighborhood to live in. They described him as polite and peaceful.

“My brother had taken him in to give him a second chance, try and turn his life around. Teach him there’s more to life than what he’d seen: gangs, drugs, violence. You come here, you work hard, this is what you can achieve,” said Byron Gamble, 53, another of Norma Gamble’s brothers.

The boy, whose name was not released because of his age, was booked and taken to Central Juvenile Hall, where he is being held without bail.

When Norma Gamble saw the scene in the bedroom Sunday night, she immediately called police and applied pressure to the girl’s wound.

The other teenage girl ran down the stairs. Gamble’s niece later found her sitting in the middle of the street.

The suspected shooter was on his way out of the house when police arrived. They stopped him and handcuffed him, Gamble family members said.

About a dozen relatives of the dead girl arrived at the home. Gamble said the girl’s mother was frantic.

“Do you know my daughter is only 15?” Gamble recalled the mother saying. “And now she’s dead.”

On Monday morning a large bloodstain remained on the bare mattress in the boy’s room. Gamble had thrown away the linens because they were soaked.

Robert.faturechi@latimes.com

Andrew.blankstein@latimes.com

Times staff writer Sarah Ardalani contributed to this report.