Waukegan police and fire crews responded to a crash just after 8:10 a.m. Wednesday in the area of Sunset and Kellogg avenues.
When first responders arrived, they found two vehicles in the roadway that had collided head-on, the coroner's office said. There were multiple victims, including the 4-year-old, who was in a booster seat.
He was taken to Vista Medical Center East with critical, life-threatening injuries and died in the emergency room, the coroner's office said.
He has been identified as Kaleb Hardge of Zion.
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All other victims were taken by ambulance to local hospitals, but officials did not provide any information on their conditions or how many others were injured.
Preliminary investigation shows the driver of one of the vehicles involved crossed the center line for an unknown reason, causing the crash, officials said.
The boy was in the other vehicle.
An autopsy performed Wednesday showed he died from blunt force injuries suffered as a result of the crash.
The Waukegan Police Department's Major Crash Unit continues to investigate.