He said the boy shot the teenager, a student at Tupelo Middle School in MS, in the back of the head and that the bullet entered her brain.
The pre-teen was rushed to Le Bonheur's Children's Hospital in Memphis and her condition is not known at this time.
Investigators say they're unsure how the boy gained access to the handgun, the Clarion Ledger reports.
Police are still investigating the circumstances of the shooting, including how the boy got hold of the weapon.
'I don't know if he knew exactly what this would do.
Speaking to USA Today, Sheriff Cantrell said: "He's just 9".More news: Drake takes on gamer in Fortnite battle with 600000 people tuning in
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The boy had grabbed a gun on Saturday (March 17) after his sister would not give up the controller. I can't answer that.
According to Cantrell he has never dealt with such a case previously and doesn't know what kind of consequences the boy will face. "I do know its a tragedy".
The mother of the siblings was reportedly in the other room feeding lunch to her other kids when White's brother killed her. "We want to do what's right and we're going to get it right".
White was a student at Tupelo Middle School, and the school district is offering counseling and bereavement support to any student or staff who needs it.
The sheriff made a call out to the public to send thoughts and prayers to the child and his family.
Monroe County is in northeast Mississippi.