Connie Villa, 35, was arrested Sunday on suspicion of one count of first-degree murder and four counts of attempted murder in the attack on Christmas in Casa Grande, south of Phoenix.
According to police, the ex-husband, Adam Villa, called 911 after fleeing the apartment. Responding officers found the body of 13-year-old Aniarael Macias. Connie Villa had stab wounds which were believed to have been self-inflicted.
Autopsy and toxicology results are pending on the girl, but police said they believed she was poisoned.
When officers took Connie Villa into custody, she was holding a knife to her chest and had her three youngest children at her side, police said. Police said the children, ages 3, 5 and 8, are in good condition but had trace amounts of opiates in their systems.
The children were placed by Child Protective Service's with Adam Villa's family. He was not the father of the 13-year-old, police said. Connie Villa was arrested Sunday after she was released from a Phoenix hospital.
It's not immediately known whether Connie Villa has an attorney. A possible motive hasn't been disclosed.