A Tucson couple face murder charges after an autopsy revealed their baby girl's death was caused by malnutrition.

Authorities arrested 26-year-old Terri Sullivan and 27-year-old Scott Sullivan after their 6-week-old child Kimberlie died Aug. 30.

Tucson police say the Sullivans are charged with first-degree murder. Initially, they were charged with felony child abuse.

A Pima County Medical Examiner's report says the baby weighed 5.8 pounds at death and 7 pounds 12 ounces at birth.

Forensic pathologist Dr. Eric D. Peters wrote there were no drugs in the "emaciated" baby's body and "no evidence of recent injury."

Tucson police say Terri Sullivan told detectives she had been starving her baby since the child was about 11 days old. She gave no reason for her alleged actions.