HUMBOLDT, Tenn. — A 12-year-old girl in Tennessee has been charged with murder, accused of smothering her 8-year-old cousin as the younger girl slept.

A relative said they had been arguing over an iPhone.

A security camera recorded the killing, inside the bedroom they shared on July 15 in Humboldt, Tennessee, the county prosecutor said.

The victim’s mother, Rayana Smith, identified the 8-year-old as Demeria Hollingsworth, according to WREG.

The recording shows the older child using bedding to suffocate her cousin as the younger girl slept in the top bunk, Gibson District Attorney Frederick Agee’s statement said. After the child died, “the juvenile cleaned up the victim and repositioned her body,” Agee said.

A relative told WREG-TV in Memphis that the girls had been arguing over an iPhone after coming from out of town to stay with their grandmother.

The girl was charged with first-degree murder and tampering with evidence after authorities obtained the video on Wednesday.

“I consider this to be one of the most disturbing violent acts committed by either an adult or juvenile that my office has prosecuted,” Agee wrote in the statement.

He said he would petition a judge to prosecute the girl, who turns 13 later this month, in adult court, which would allow for “a lengthier sentence, whether that will be through incarceration or supervision with court-ordered conditions.”