WASHINGTON, March 19 (Xinhua) -- A nine-year-old boy shot his 13-year-old sister to death after they argued over a video game controller late Sunday night, according to local authorities of Mississippi.

The girl from Monroe County was shot in the back of her head, and the bullet went through her brain.

"He wanted the controller, and I guess he knew where to find the gun," Sheriff Cecil Cantrell told CNN.

The sheriff assumed that the boy saw some shooting scene on TV or video games. But it's not clear how the boy gained access to the weapon, and how much knowledge he had about the danger of guns.

The case is still under investigation and authorities have not filed charges.

"This is all new ground for us, we've never dealt with a kid shooting at age nine," Cantrell said. "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."

Nearly 1,300 children died and 6,000 were treated for gunshot wounds on average each year in the United States, according to a study published in the Journal of Pediatrics in 2017.

The study revealed that U.S. boys, older children, and minority children are disproportionately affected by gun violence. Boys accounted for 82 percent of all child firearm deaths.