The officer who fatally shot Marcellis Stinnette and wounded Tafara Williams following what authorities have described as a traffic stop late on Tuesday committed “multiple policy and procedure violations”, Waukegan police chief Wayne Walles said in a brief statement.
No other details, including the officer’s name, have been provided.
The announcement came shortly after Lake County’s chief prosecutor announced the FBI will join Illinois State Police in investigating the shooting.
Lake County State’s attorney Michael Nerheim has asked the US justice department to review the circumstances surrounding the shooting.
“I am confident in the work being done by the Illinois State Police and welcome the assistance of the FBI,” Mr Nerheim said in a statement.
“As I have said before, once the investigation is concluded, all the evidence will be reviewed and a final decision will be made with respect to any potential charges.”
Waukegan police have said Ms Williams was driving and Mr Stinnette was a passenger in a vehicle that fled a traffic stop late on Tuesday before the vehicle was later spotted by another officer.
Police said that as the second officer approached, the vehicle started moving in reverse and the officer opened fire.
No weapon was found in the vehicle.
The officer who shot the couple is Hispanic and had been with the Waukegan police Department for five years.
Police said the officer who conducted the original traffic stop was white.
During a demonstration on Thursday, Clyde McLemore, head of the Lake County chapter of Black Lives Matter, called for a federal investigation.
Activists and relatives of the couple also have demanded the release of police video of the shooting, which authorities say has been turned over to investigators.
Activist Chris Blanks said the video is particularly important because the police version of events and the version Clifftina Johnson said her wounded daughter, Ms Williams, shared with her, appear to contradict each other.
Ms Johnson said her daughter told her that Ms Williams and her boyfriend had done nothing to provoke the officer.
Mr Nerheim urged calm while the investigation takes place and pledged transparency.