A Utah man serving time in a secure juvenile facility for a murder and rape he committed when he was under 18 was charged Thursday with assaulting a fellow inmate.

Jayden Sterzer, who was 15 years old when he killed 12-year-old Kailey Vijil in 2015, was booked into Salt Lake County jail after he was charged with punching, shoving, head butting and putting another inmate in a choke hold.

(Courtesy Salt Lake County jail) Jayden Sterzer

Court documents allege Sterzer, who is now 19, attacked the inmate Aug. 14 after being let out of his room. He was charged Wednesday with a third-degree felony count of assault by a prisoner.

Sterzer was serving time in the juvenile facility as part of a “blended” plea, where he admitted to a charge of rape of a child in juvenile court and pleaded guilty to murder and sexual abuse of a child in adult court. He was slated to stay at the juvenile facility until he turned 21 — or was otherwise released, if that came first — and would be sentenced to adult prison.

Salt Lake County District Attorney Sim Gill said this charge, and Sterzer’s release from a juvenile secure facility, would trigger Sterzer’s adult sentencing, where he faces a mandatory prison term of at least 15 years and could spend up to life in prison.

Sterzer’s original charges stem from Kailey’s July 16, 2015 death.

Sterzer had knocked on the Vijil’s door and asked Kailey to help him find a lost cat. He then took Kailey to an overgrown pasture about a half-mile from her family’s West Valley City home and sexually assaulted the girl. He then strangled her to death with his tank top.