Jazzmin Davis was found dead at the Antioch home of her foster mother Shemeeka Davis, who is also her aunt.
Davis has been charged with her murder.
Antioch police said it was the worst case of child abuse they'd seen.
An investigation by the Contra Costa Times found the child welfare worker who was assigned to Jazzmin violated state law when she did not obtain regular reports from the girl's doctors and therapists.
