The boy was found guilty of first-degree murder for shooting MaKayla Dyer in the chest with a shotgun after she refused to let him play with her puppy, according to a sentencing document obtained by The Newport Plain Talk and posted online by CNN affiliate WATE
He will remain incarcerated until his 19th birthday, according to the court document.
The attorneys for the prosecution and the defense would not comment to CNN, but the boy's great-grandmother, Dianna Houchins, told CNN that an appeal is planned.
According to the sentencing document, MaKayla and her sister were talking to the boy from outside the mobile home in rural White Pine, Tennessee, where the boy was staying, on the evening of October 3.
The adults of the home were inside watching TV in another room, according to the court document, which is dated February 2.
Speaking to the girls through a window of the mobile home, the boy asked MaKayla if he could play with her puppy, but she refused, the document states.
The boy took a BB gun and a shotgun from a closet, and pointed the shotgun through the window of the home -- down at MaKayla -- and fired, hitting her just above her heart, the document states.
MaKayla's obituary says she had turned 8 just three weeks before her death.
Houchins told WATE after the shooting that she did not believe her great-grandson pulled the trigger. She believed that the autopsy would show that someone at ground level shot MaKayla.
The shot, however, came from the position where the boy was standing, according to the court document, and hit the girl "at a downward trajectory."
The boy, according to the document, "rendered no aid to the victim. The victim was unarmed," he is "guilty, beyond reasonable doubt, of the unlawful, intentional, and premeditated murder of Mckayla [sic] Dyer."
In an interview with WATE in the days following the shooting, MaKayla's mother, Latasha Dyer, wept while she said, "When we first moved to White Pine, the little boy was bullying MaKayla and I had to go to the principal about him, and he quit, for a while. Then all of the sudden yesterday, he shot her."
The shooter's great-grandparents vehemently denied that the boy had been bullying MaKayla.