The officer, identified as 30-year-old Jaimie Cox, made a traffic stop in the area of State Street and Dawn Avenue at about 1 a.m. A short time later, Cox made a brief radio transmission calling for assistance.
Officers arrived on the scene and found a single-vehicle car crash about two blocks north of the initial traffic stop. Officers also located Cox, who was critically injured and administered first aid to Cox.
Cox was transported to OSF St. Anthony Medical Center, where he died later Sunday morning, police said. A man was found dead inside the vehicle, police said. He has been identified as 49-year-old Eddie Patterson.
Rockford Police Chief Dan O'Shea said that the officer had been involved in a scuffle and became entangled in the vehicle. Cox fired his weapon, but it was unclear if either he or Patterson was hit.
Cox had been with the Rockford Police Department since December 2016, O'Shea said. Previously, he was with the Illinois Department of Natural Resources.
Police continue to investigate the incident. O'Shea said there is not a threat to the public.
Governor Bruce Rauner released a statement saying, "We are deeply saddened by the death of Officer Jaimie Cox. He paid the ultimate price while working to keep our communities safe. My thoughts and prayers are with his family as they find a way to cope with this tragic loss."