The Cook County medical examiner's office says baby boy Aaron Sweatt of Park Forest suffered multiple injuries, blunt force trauma and child abuse.
The Chicago Tribune reports that authorities say Chicago Heights police received a call Thursday from the Illinois Department of Children and Family Services about the boy, who was being treated at a hospital.
The 3-month-old was found unresponsive late Friday inside a Chicago Heights residence. He was rushed to the hospital and pronounced dead early Saturday.
The medical examiner says the baby was pronounced dead Friday afternoon.
Authorities with the Illinois Department of Children and Family Services say they are investigating the case.
The child's mother tells ABC7 that her son was not in her care at the time of the incident and that she has no knowledge of what happened to her son.
The Associated Press contributed to this report.