A Colorado judge sentenced a teenager to life in prison on Monday after the teen pleaded guilty to the 2017 murder of a ten-year-old girl.

The judge sentenced Aidan Zellmer, 17, to life in prison with the possibility of parole in 40 years in connection with the June 2017 murder of ten-year-old Kiaya Campbell, who was the daughter of his mother’s boyfriend, KDVR reported.

Zellmer, who was charged as an adult, was 15 years old at the time of Campbell’s murder. Campbell went missing after the two traveled to a shopping center from their father’s home.

Zellmer and Campbell had left the house to steal candy and Pokemon cards, according to an arrest affidavit.

The arrest affidavit noted that Zellmer used a metal pipe to hit Campbell’s head five times before she dragged her unconscious body into a ditch and left her to die.

Officials issued an Amber Alert and launched a massive search effort to find Campbell after she went missing. Neighbors discovered her brutally beaten body in a ditch the next day, the Denver Post reported.

Zellmer initially told investigators that Campbell went missing after two had been separated during a rainstorm.

An autopsy report later confirmed that Campbell died of blunt-force trauma to her head and suffered wounds to her fingers. Officers later arrested Zellmer after officers discovered the blood and DNA on his shoes linked him to the crime.

Zellmer took a plea deal in February where he agreed to plead guilty to one count of first-degree murder.

“In 27 years as a prosecutor, I have never seen a 15-year-old commit such a violent act,” 17th Judicial District Attorney Dave Young said.