Authorities said a dead person was also located inside the vehicle of the vehicle crash.
Shortly after, investigators say the officer made a radio transmission saying he needed assistance.
They also discovered a auto crash about two blocks away from the initial traffic stop with a dead body inside.
Cox was rushed to OSF Saint Anthony Medical Center in Rockford, where he was pronounced dead, O'Shea said.More news: Trump campaign aide Cater Page met Russian officials in 2016
Following Cox's death, local police escorted his body to the county coroner's office.
Around 1:20 a.m., Rockford police posted on Twitter and Facebook that the department was "on scene of a shooting involving an officer".
O'Shea says Cox became "entangled" with the vehicle and fired a shot, but he declined to say how either man died or to provide additional details. "The area is still blocked off of the incident, the detectives are out there working".
He joined Rockford PD in December 2016.More news: Donna Brazile Reportedly Pondered Replacing Clinton With Biden in 2016
Investigators said they believe Cox was involved in a scuffle during the traffic stop "that resulted in the officer becoming entangled by the suspect vehicle".
He beforehand filled in as an officer for the Illinois Department of Natural Resources, O'Shea said.
O'Shea identified the officer as 30-year-old Jaimie Cox. He was a graduate of Hononegah High School and Northern Illinois University.
A Rockford Police officer was shot and killed early Sunday, Nov. 5, on the city's east side. Cox was transported to a hospital, where he died later Sunday morning.More news: West Ham's new manager revealed during first half v Liverpool
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