The preteen faces first-degree murder charges in the death of MaKayla "BooBah" Dyer, 8.
He fired at her from his house Saturday, killing the girl who was in her yard, Jefferson County Sheriff Bud McCoig told The Washington Post.
The boy used his father's 12-gauge shotgun, which he got from an unlocked closet,
the sheriff told the paper.
He is being held on a first-degree murder charge in a juvenile detention center. The center's superintendent, Richard Bean, said the boy is the youngest held there on a murder charge in his 44-year career. Bean described him as "very tiny," weighing about 55 pounds.
He is in a regular "pod" that holds up to 16 other juveniles between ages 12 and 17.
'Most pitiful case'
The incident started when the boy asked to see the girl's puppy, McCoig said.
When she said no, the boy got a gun and shot her dead Saturday, according to the sheriff.
The Jefferson County District Attorney's Office will determine whether the boy should be tried as an adult.
He is being detained until a hearing on October 28, his public defender, Edward Miller, told CNN affiliate WATE
. He declined to comment further.
"It is the most pitiful case on both sides," Bean said.
'I want her back home'
MaKayla died three weeks after her 8th birthday.
Her mother, Latasha Dyer, choked back tears as she talked about her daughter.
"I want her back home, I want her back in my arms," she said. "This is not fair."
Her death has rattled the small town of White Pine, which is 40 miles east of Knoxville.
"She was a lovable, little, adorable and typical 8-year old girl that enjoyed life and loved everybody," great-uncle Kenneth Norton told CNN. " She didn't know (any) strangers ... and anytime that you were doing anything ... she wanted to be a part of it."
She will be buried Thursday at 2 p.m. in Luttrell, Tennessee.