A Colorado teen will be tried as an adult in the murder of a 10-year-old girl who reportedly disappeared on a walk with him.

Aidan Zellmer, 15, was arrested in June after the body of Kiaya Campbell was discovered face down in the grass at a Thorton park, the Denver Post reported. Local news outlets identified Zellmer as the son of her dad’s live-in girlfriend.

A judge ruled Tuesday that Zellmer’s case will be transferred to adult court.

“We obviously think it was the appropriate decision,” Adams County District Attorney Dave Young told the Denver Post.

Zellmer told investigators that the pair was separated walking from her dad’s home during a thunderstorm — although some disputed whether it rained the evening she vanished.

Campbell’s autopsy report revealed she was brutally beaten and there were signs of blunt force trauma to her head, which resulted in skull fractures.