(CBS) — The medical examiner in Cook County says the death of a 2-year-old DeKalb boy was a homicide.

The case began last Friday night, when police in DeKalb were alerted by hospital staff that a 2-year-old boy, Khalil Body, was being transferred to Lurie Children’s Hospital in Chicago because of a severe brain injury.

Now, Khalil has died at Lurie. His death has been ruled a homicide.

Police in DeKalb say the girlfriend of the boy’s biological father was arrested and charged with Khalil’s murder.

Police say 21-year-old Alexandra Hoyle slammed the boy’s head into the hardwood floor of a home in DeKalb.

Her bond was set at $5 million.