The mother of the girl, 35-year-old Collette Bancroft, was arrested by Joliet police for possession of controlled substance.
Officer responded to a home at 8:24 a.m. Sunday in the 1500 block of East Washington Street in Joliet. They found a 12-year-old girl unresponsive in a second-floor bedroom and provided medical assistance, including Narcan.
The girl was taken to Silver Cross Hospital for overdose treatment. She was later transported to Comer Children's Hospital in Chicago, where she was pronounced dead after 5 p.m. on Wednesday.
Investigators learned the girl had ingested an unknown amount of suspected heroin and fentanyl, Joliet police said. Suspected heroin and fentanyl was also found in Bancroft's purse in the bedroom.
Bancroft has been released from custody with a notice to appear in court.
Illinois DCFS has been notified about the death, police said.
No further information was immediately available.