A nine-year-old boy shot and killed his sister in a row over a games console controller.
The 13-year-old girl died in hospital after being shot in the head in Mississippi in the US.
The boy grabbed a gun after his sister refused to give up the video game controller, according to Monroe County Sheriff Cecil Cantrell.
He then shot the girl in the back of the head causing the bullet to enter her brain, he added.
The girl, named in US media reports as Dijonae White, was taken to hospital in Memphis, Tennessee, but later died.
Authorities said they do not yet know how the firearm used in the shooting had been accessed.
The sheriff added that the circumstances of the shooting are still being investigated.
“This is all new ground for us, we’ve never dealt with a kid shooting a kid at age 9,” Sheriff Cantrell told the Clarion Ledger newspaper.
“I don’t know if he knew exactly what this would do. I can’t answer that. I do know it’s a tragedy.”
He added: “We don’t know yet what kind of charges or if charges will be pressed. We want to make sure we’re doing everything correctly.
“That’s why I’m not too fast to say anything because there are juveniles involved. We want to do what’s right and we’re going to get it right.”