A Colorado teen who sexually assaulted and murdered a 10-year-old girl will spend the rest of his life behind bars. The 17-year-old, Aidan Zellmer, was tried as an adult in the 2017 murder of 10-year-old Kiaya Campbell and was sentenced to life in prison at the Adams County District Court on Monday, March 11, after previously pleading guilty to his charges.

According to Daily Mail, Zellmer was due to be charged with first-degree murder with intent and after deliberation, as well as first-degree murder of a child under the age of 12 by a person in a position of trust. He later struck a deal with prosecutors in January which saw them drop one of the charges of first-degree murder and three charges of sexual assault, but was still reliably informed he would receive a life sentence.

MEA WorldWide (MEAWW) previously reported that Campbell — who was the daughter of the woman Zellmer's father was dating — was first reported missing on June 8, 2017. An AMBER alert was issued, with 23 rescue and law enforcement agencies working together to locate the 10-year-old girl.

It would be a neighbor who ultimately discovered Campbell's body in a ravine which was 1.5 miles from where she disappeared, with a medical examination revealing that the preteen's body showed signs of severe trauma and that she had possibly been sexually assaulted.

It was initially believed that the 10-year-old had gone out to a shopping center with a friend and had then been separated during a subsequent rainstorm. However, it later emerged that she was last seen with Zellmer, who was eventually arrested and charged with first-degree murder in connection to her death.

Zellmer was sentenced to life in prison (Source: Thornton Police Department)

Adams County Juvenile Court Judge Priscilla Loew ruled in the favor of district attorneys who were pushing for Zellmer to be tried as an adult due to the gruesome nature of the crime, with this week's ruling finally bringing the case to a close.

Speaking about Zellmer, 17th Judicial District Attorney Dave Young said, "In 27 years as a prosecutor, I have never seen a 15-year-old commit such a violent act. He wanted to do this… His actions certainly dictate that he intended to kill her and it took quite an effort to do it. And then to conceal it, not only to hide the body but mislead. Certainly indicates that his brain was developed and he knew what he was doing."

Adams County District Attorney said Zellmer would be transferred to an adult prison after he served two years of his sentence in a juvenile detention facility. The teen will be eligible for parole after serving 40 years of his sentence because he was a minor when he committed the murder. Reductions in time for good behavior, as well as a program for juvenile killers that allow them to apply for parole after serving 25 years, could mean he's released sooner than that.