If convicted, Martwane Mackey faces a life sentence. Maricopa County prosecutors have yet to decide whether to seek the death penalty.

In addition to blunt-force trauma to the girl's head and torso, police noticed bite marks on her body, which Mackey admitted causing.

The unemployed Mackey called police about 2 p.m. June 25 from a payphone down the street from the apartment he shared with the girl's mother at 1601 West Missouri Avenue in Phoenix.

When police arrived at the payphone, they found Mackey holding the toddler. She wasn't breathing.

Rescue crews tried to revive the girl, but they were unsuccessful and she died.

Because his girlfriend works and he doesn't, Mackey was tasked with watching the girl and her 1-year-old sister that morning.

According to court documents obtained by New Times, Mackey admitted to abusing the girl and her younger sister, who was found to have similar injuries but is expected to survive.

Mackey told police that earlier in the day, the 3-year-old had a "choking incident" that left her unconscious and not breathing at times. Rather than call 911, Mackey thought biting the girl on the face and torso might bring her back to consciousness, he later told police.

Mackey had no explanation for the bite marks on the girl's younger sister, though.

While Mackey admitted to abusing the girl, he claimed he didn't kill her. He told police he suspected she may have hit her head on the toilet, or something.

In addition to the murder charge, Mackey was charged with two counts of child abuse. He was booked into a Maricopa County Jail on $1 million bond. His next court date is scheduled for August 24.

--James King contributed to this post