A TEEN has been jailed for life after reportedly sexually assaulting and beating his 10-year-old stepsister to death.

The crimes were so shocking the judge in the case said he had "never seen such violence from a 15-year-old".

4 Aidan Zellmer, age 15 at the time of his arrest, is now facing life in prison after being put on trial as an adult Credit: Thornton Police DepartmentKiaya

The Colorado teen was slapped with a sentence of life in prison, but because of his age, he may be out within 30 years.

Aidan Zellmer, now 17, pleaded guilty last month to first-degree murder over the 2017 death of Kiaya Campbell.

He was initially facing sexual assault charges as well, but these were dropped as part of a plea deal.

District Attorney Dave Young echoed the judge, and said in his 27 years as a prosecutor, he had 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 District Attorney Dave Young

He said: “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.”

Kiaya disappeared on the night of June 7, 2017, while walking to the store with Zellmer.

Her body was found beaten to death in a drainage ditch the following day a mile from the home in Thorton where Kiaya's father lived with his girlfriend and her two sons.

Zellmer initially told investigators that he and Kiaya were separated when he started running to escape a rainstorm.

The first clue something was wrong came from neighbours, who said there had been no rain that night.

A 10-year-old's life is gone, and her parents are going to be without her for the rest of their lives District Attorney Dave Young

Because Zellmer was only 15 at the time of the murder, he will automatically be eligible for parole after serving 40 years in prison, but could get out in 30 years with reductions in time for good behaviour.

He will serve the first two years of his sentence in juvenile detention before being transferred to an adult prison when he turns 18.

The Denver Post reported that a Zellmer's defense attorney was visibly emotional at the sentencing, and spoke about Zellmer’s mental health issues, troubled family history and alleged abuse.

But Young and family members dismissed Zellmer’s past as an excuse for the killing.

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The prosecution also reportedly played a video with clips of Kiaya dancing and singing, and her mother Chantel Campbell testified that she took the video “everywhere” so she could watch it when she misses her daughter the most.

Zellmer reportedly stood emotionless through the sentencing.

Young told CBS: “A 10-year-old's life is gone, and her parents are going to be without her for the rest of their lives.”

The murder weapon was never recovered, but Zellmer had reportedly told others he used a metal object to beat Kiaya, Young said.

4 Victim Kiaya Campbell was reportedly struck five times in the attack that killed her Credit: Thornton Police DepartmentKiaya

4 Judicial District Attorney Dave Young said he had never seen such violence from a 15-year-old in his 27 years as a prosecutor Credit: 9NEWS

4 Kiaya went missing on the night of June 7, 2017, while out on a walk with her step-brother Credit: Thornton Police DepartmentKiaya

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