Inside the crashed vehicle, officers found 49-year Eddie Patterson dead. A short time later, Cox called for assistance.
He was transported to OSF Saint Anthony Medical Center in Rockford, where he died as a result of his injuries.
Rockford officers responding to the scene found Cox, who was critically injured, near a crash about two blocks north of the initial traffic stop, O'Shea said.More news: NFL Week 9: Keys for Giants vs. Rams
Rockford Police officer was discovered dead close to a traffic crash and a man was dead in a vehicle early Sunday following a conceivable shooting, police boss Dan O'Shea said.
Following Cox's death, local police escorted his body to the county coroner's office.
Rockford Police say they are holding another media briefing at at noon at the police administration headquarters, 557 South New Towne Drive.More news: Church tragedy: More than 20 dead in Sutherland Springs shooting
"The examination is as yet continuous and exceptionally dynamic", O'Shea said.
Rockford police have not commented on if the suspect did in fact take Cox's gun. "To the extent suspects, we don't feel people in general is in risk at the present time".
A Rockford police officer was killed overnight in the line of duty.More news: Sadio Mane helps Liverpool down West Ham
"Our thoughts and prayers are with Jamie, Jamie's family and his wife", O'Shea said. He received an honorable discharge from the U.S. Army National Guard in 2010, and then worked for the Illinois Department of Natural Resources' law enforcement division before joining Rockford's force. "It is believed that officer Cox fired his weapon during this incident".