Arnold said he hoped to file formal charges in the case this week.
The Tabbs already were being held without bond in the Delaware County Jail on preliminary charges of neglect of a dependent causing serious bodily injury. Jail records didn't indicate whether they have an attorney.
A deputy coroner has said the girl, identified only by the initials M.P., was from Haiti and that the birth mother still lives there.
Hahn said he hasn't determined the exact cause of the girl's death, even after an autopsy Monday. The girl was found dead Saturday night.
"She's got very obvious injuries that would've required medical attention," Hahn said.
The Star Press reported court documents released Monday indicated the victim had, among other injuries, open sores and swelling to her hands, an open blister on her right ankle, and "odd markings" on both of her nipples "which appear to have been pinched . by a pair of pliers or another rectangular object."
Police Chief Steve Stewart called it "one of the most horrific cases I've been involved with in my 28 years with the Muncie Police Department."