The body of Katelynn Arnold, 9, was discovered Thursday night outside her home in Ragland, a rural town about 40 miles northeast of Birmingham, said St. Clair County Sheriff Terry Surles.
The cotton rope from an old tire swing was deliberately tied in a simple knot around the girl's neck, but was not a hangman's noose, the sheriff said. St. Clair County Coroner Dennis Russell said an autopsy found the cause of death was homicide due to ligature strangulation.
The boy and girl were living with their aunt and uncle as part of a foster home situation, Surles said.
The girl was riding her bicycle Thursday evening and her aunt noticed she was missing. She drove around looking for the girl for about 45 minutes and came home to find her hanging from a tree outside the home.
A call came into the sheriff's department about 8:10 p.m. and by the time investigators arrived, rescuers had removed the girl from the tree. She was pronounced dead at a hospital.
The boy faces juvenile charges so his name was not released. Authorities could later decide to charge him as an adult.
After the girl was found and taken to the hospital, the boy went to bed, Surles said. He was arrested between 2 and 3 a.m. and admitted to killing the girl, Surles said. He said the boy told investigators a motive but Surles did not release it.
The names of the aunt and uncle were not released and authorities did not have information on where the children's parents might be. It was not clear whether the state had placed the children in their care or if the parents had sent the pair to live there.
Ragland Elementary School planned to have grief counselors available Friday for classmates of the girl.