Does a police car need to have a special license plate?

I know that all marked police cars in my state (Illinois) have special green (some sort of government plates) or special plates with MP (Municipal Police) on them. I also have noticed that all unmarked and undercover police cars (other government such as state secret service also have these distinguishing plates while having no other conspicuous markings). I simply umed all government/police vehicles had to have these special plates to show they are in fact a police officer and not an impostor, Today however I saw a police car in my town with a normal civilian plate, I would not have known this was a police car until I saw a uniformed officer enter the vehicle and see the strobe lights hidden inside the car.