THORNTON, Colo. — The 17-year-old who pleaded guilty to murdering a young girl in 2017 was sentenced Monday morning to life in prison with the possibility of parole.

Around 11:50 p.m. on June 7, 2017, a woman called the Thornton Police Department to report that her 15-year-old son Aidan Demmie Zellmer, and 10-year-old stepdaughter Kiaya Campbell had left their house in Thornton around 7:30 p.m. to go to a store and hadn't returned. Neither had a phone and the mother said they had been gone a long time.

Officers started to search for the pair. Campbell's biological father helped with the search, and found Zellmer walking on Fairfax Street. When Zellmer talked with police, he said they had gone to the store to steal Pokemon cards, but were unable to because the cards were locked. They left to go home, where Zellmer grabbed a backpack with waters, before heading back out to another store. While walking to the second store, it started to rain so they turned onto a walking path east of Colorado Boulevard, Zellmer told police.

He said they got in an argument as they waited for the rain to let up — Zellmer wanted to wait out the store and Campbell wanted to go home. They argued for a little while before deciding to stay in that location. When the rain let up, they started to walk back home. As they walked, Zellmer turned around and didn't see Campbell, he told police. He searched for her, but after 45 minutes, he still hadn't found her. Around that time, Campbell's father found him on Fairfax Avenue.

As police talked with Zellmer, they said this story did not add up, but the teenager insisted that was what happened. When asked if he did something to Campbell, he said he had not. Police obtained surveillance video that did not match Zellmer's story.

On June 8, 2017, police issued an AMBER alert for Campbell . More than 20 police agencies and hundreds of volunteers searched for her.

That same day, as Zellmer was in an interview with police, Campbell's body was found by a resident in the greenbelt area north of 128th Avenue between Jasmine Street and Jasmine Court. She had been beaten to death with a metal pipe.

When police told Zellmer during the interview that they had found Campbell's body, he became inconsolable, according to an affidavit. He said he had left her because she was being annoying. Both Zellmer and his mother consented for him to have a sexual assault examination and Zellmer was placed with the Department of Human Services at his mother's request.

According to Campbell's autopsy, she had died from blunt force trauma to the head. A forensic pathologist said she had been beaten in the head at least five times. She had other wounds, including some defensive wounds.

Zellmer's clothing and shoes were collected for testing. A presumptive serological examination found blood on both of his shoes.

Based on the test results, he was arrested on a first-degree murder charge three days after Campbell went missing.

In February, he pleaded guilty to first-degree murder and on Monday morning, he was sentenced to life in prison with the possibility of parole after 40 years.

District Attorney Dave Young discounted theories about Zellmer's brain development as an excuse for the crime.

“Fifteen-year-olds make bad decisions but they don’t commit cruel, heinous crimes like this,” he said. “After striking her five times win the head with a metal pipe, he left her in a field. When he talked to police, he was calm, cool and collected. He misled and deceived police who were looking for her. This is a rare case. Thank God we don’t have other 15-year-olds doing what he did."

When she imposed the life sentence, Adams County District Judge Sharon Holbrook told the family, “Kiaya will not be forgotten."

TIMELINE:

Wednesday, June 7, 2017 — Campbell leaves her home with a 15-year-old friend at about 7 p.m. to head to a shopping center.

Wednesday, June 7 — At 11:52 p.m., Thornton Police Department officers rushed to Forest Drive when family members reported Campbell as missing. Originally, police said she had been separated from the 15-year-old friend in a rain storm near the 12900 block of Colorado Boulevard.

Thursday, June 8 — Through the night and early into the morning of June 8, police continued a wide search that included bodies of water.

Thursday, June 8 — AMBER Alert activated at 1:37 p.m. as concern grows and search intensifies. Police gave a new location where she had last been seen at 12466 Forest Drive.

Thursday, June 8 — AMBER Alert canceled after a resident found Campbell's body at 5:30 p.m.

Friday, June 9 — Coroner identifies the body as that of 10-year-old Campbell on Friday afternoon. The coroner affirms the cause of death was not accidental.

Friday, June 9 — At 7 p.m., Thornton Police Department officials deem the investigation a homicide.

Friday, June 9 — Hundreds of pieces of evidence have been collected, with police announcing they will be sorted and analyzed.

Saturday, June 10 — At 11:38 p.m., police arrested a 15-year-old juvenile male for investigation of first-degree murder.

Sunday, June 11 — Police release details of the arrest to the public, declining to name the teen.

Monday, June 12 — The 15-year-old appeared at a 1:30 p.m. hearing to decide where the teen will be held. The judge closed the hearing, denying media access.

Wednesday, June 14 — First-degree murder charges filed in juvenile court, however DA announces desire to see adult charges filed.

Tuesday, Jan. 23, 2018 — The case is moved to adult court so that Aidan Zellmer can be tried as an adult.

Friday, Feb. 1, 2019 — Zellmer pleads guilty to a charge of first-degree murder.

Monday, March 11 — Zellmer sentenced to life in prison with the possibility of parole.

