It was always known that Jayden Sterzer would one day go to the adult prison for raping and murdering his 12-year-old neighbor.

But that day came much sooner than initially expected.

A 3rd District judge on Monday sentenced Sterzer to spend a 15-year-to-life sentence at the Utah State Prison, a decision that came after authorities say Sterzer assaulted a staff member while he was in a juvenile detention center.

Sterzer, now 19, admitted last December that he killed Kailey Vijil in 2015, after he knocked on her door and asked her help to find a lost cat. The then-15-year-old boy brought Kailey to an overgrown pasture about a half-mile from her family’s West Valley City home, where he sexually assaulted then strangled her.

Kailey Vijil courtesy of the Vijil family

His admissions were part of a “blended plea,” where he admitted to charges in both juvenile and adult court.

Sterzer's plea was intended to allow him to stay in the juvenile facility for three years to receive treatment until he turned 21 — unless the Youth Parole Authority decided they can no longer help him.

That happened after authorities filed charges against Sterzer for allegedly assaulting a detention center staffer in August. The transfer to adult prison came about a year earlier than expected for Sterzer, who turns 20 in a few days.

Third District Judge Paul Parker on Monday sentenced Sterzer to a 15-year-to-life sentence for murder, and a one-to-15-year term for sexual abuse of a child. The judge ordered the sentences to run concurrent to each other.

Defense attorney Michael Sikora told the judge that Sterzer has struggled his whole life, and it wasn’t until after he was charged in Vijil’s death that he was evaluated and diagnosed with fetal alcohol disorder.

Sikora said Sterzer was in a mental health program at age 4, started taking medication when he was 8 years old, and by age 9 had exhibited concerning behaviors like smearing feces and hoarding food. He attempted suicide at age 11, the defense attorney said, and was bullied by his peers.

“In this case, it was one child killing another child, and I think it’s important to remember that,” Sikora said. “It’s easy to forget that Jayden was a 15-year-old child when this occurred. Kailey had a bright future, no doubt about it. Jayden, to be candid, not so much.”

But Kailey's father, Orlando Vijil, had little sympathy for his daughter's killer. Sterzer was no longer a young boy, Vijil told the judge — he was a man.

And Vijil said he still holds so much anger for what Sterzer did.

“Every time I get a memory of my daughter, it hurts,” Vijil said. “He promised me he was going to change his life, and he couldn’t do that. Not even for one or two years. I want to rip his goddamn head off right now.”

Sterzer’s sentencing in the adult court mirrored that which came a year ago, when a similar hearing took place in the juvenile court before he began serving his sentence there.

Both times, Kailey’s father clung to the courtroom podium and took a long pause. He stopped to take a deep breath, then another, before speaking.

A year ago, Vijil spoke about how beautiful and loving his daughter was and how much he missed her. How he had a pain that would never go away.

That pain hadn't lessened a year later, as he described how he's come to court dates for four years and has lost four jobs since his daughter was killed.

"I quit doing all the stuff I used to do," Vijil said. "I don't go out anymore because of what this guy did."

Kailey's mother, De Shawn Undergust, said her grief has become its own life sentence for her. She said she hopes Sterzer is never released from prison.

"Kailey had so much to live for and a bright future ahead of her," the mother said. "On Nov. 21, my girl would have been 17 years old. But because of what this monster has done, she will forever be 12 years old."

Sterzer hung his head throughout his sentencing hearing as Kailey’s family cried and expressed their anger toward him. He told the judge that he knew that he deserved to be punished.

"I know what I did was wrong," Sterzer said. "I'm truly sorry for what I've done."

(Rick Egan | The Salt Lake Tribune) Jayden Sterzer, right, stands with his defense attorney Michael Sikora, who was sentenced one-to-15 years for sexual abuse and 15-years-to-life for the murder of his 12-year-old neighbor, Kailey Vijil, when he was 15-years-old in 2015, in 3rd District Court in Salt Lake City, Monday, Nov. 18, 2019.
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Sterzer is expected to be in court later this week for a preliminary hearing on the accusation that he assaulted the youth staff member. He is facing a charge of third-degree felony assault by a prisoner, which carries a penalty of up to five years in prison.

Charging documents allege that on Aug. 14, Sterzer came out of his room in an "aggressive" manner and swung at a staff member, knocking the staffer to a wall.

"At one point during the physical confrontation, Jayden had the staff member in a bear hug," a police officer wrote in a probable cause statement.

The assault was not reported to police until more than a week later, according to arrest records. Sterzer was charged with assault by a prisoner, a third-degree felony.

Sterzer is one of a handful of Utah teens who have entered "blended pleas" after being accused of heinous crimes. And he isn't the first to be transferred to the adult prison early.

In 2015, then-17-year-old Aza Vidinhar was transferred to the adult prison after he assaulted another youth at a juvenile detention center. Vidinhar had admitted to fatally stabbing his younger brothers.