Family members said Tuesday they believe a teen was showing his younger brother a gun when it went off.
The younger boy, 14-year-old Damion Rolle, was killed in the afternoon incident near West 70th Street and South Yale Avenue, in Chicago's Englewood neighborhood.
Rolle's older brother, 15-year-old Devonte, was still in police custody as of 8 p.m. Wednesday, according to the boys' aunt, Janice Smith.
Smith said both teens have autism and said she believes the incident was an accident. She called her nephew a "good little boy" and said the boys' mother didn't own a gun.
"When they get mad at one another, you know how boys can be. They have fights, wrestle, tussle one another. Having a gun in the house? It was shocking," said Smith. "Where he got this gun? I just don't know."
The boys’ mother works in the kitchen at the Great Lakes Naval Base, said Smith, describing the mother as a hard-working, concerned parent.
"She didn’t even have a knife in their house because they had autism," Smith said.
Community activist Andrew Holmes said the gun was thrown out of the window and later recovered by police.
Damion Rolle was initially taken to the University of Chicago Comer Children’s Hospital in serious to critical condition, said fire department spokesman Richard Rosado. Rolle later died, police said.