A Las Vegas teenager who told police that devils encouraged him to kill his 6-year-old relative is facing a new murder charge after he allegedly stabbed his jail cellmate with a pencil Friday morning, police said.

Carl Guilford, 18, stabbed the cellmate during a fight at the Clark County Detention Center, said police spokesman Bill Cassell. Jail officials discovered the unconscious inmate minutes later while conducting a routine bed check at 1:20 a.m. The man was suffering from apparent head trauma and attempts to treat him were unsuccessful. He was soon pronounced dead.

Cassell said Guilford underwent a psychiatric evaluation after he was arrested.

"He had been deemed safe to be housed with other inmates," Cassell said.

It's unclear what started the fight between Guilford and his cellmate.

Guilford was arrested in May after he confessed to suffocating Christopher Montgomery with a blanket, according to an arrest report.

The boy's mother told police that her cousin, Guilford, was bi-polar and had stopped Montgomery from breathing in the past by shoving an object down his throat.

Other family members, including Guilford's mother, told police that Guilford had been acting strangely for months after he allegedly smoked marijuana laced with another substance. The relatives said he was a "troubled young man" who regularly spoke of spirits, according to the arrest report.

Guilford was living with his cousin, her son and her daughter at the time of his arrest and shared a room with Montgomery.

Guilford told police he was heading to the Las Vegas Strip on the night of Montgomery's death when the child walked into their shared bedroom and began complaining about the surroundings. Guilford told police he held a blanket over Montgomery's mouth to shut the boy up because he didn't want anyone else to wake up.

Guilford said when he removed the blanket after 30 seconds, the boy was no longer breathing, according to the arrest report.

Guildford told investigators he heard "devils" tell him "well done" as he left the room.

The Clark County coroner's office did not release the identity of the dead inmate and police would not say why he was in jail.

Inmates are searched for potential weapons when they are admitted to the jail, but pencils are allowed within the detention center, Cassell said.

This is the first time an inmate at the jail has been killed since 1979, when one man strangled his cellmate. That man is on death row for the crime.

Guilford was scheduled to return to court Tuesday for a competency hearing stemming from Montgomery's death.