I appologize in advance if this is not the right forum for discussing this.

In Illinois, most marked municipal police cars have a license plate with green lettering, whose first letter starts with M. However, every once in a great while I'll see a car on the highway with an Illinois license plate with green lettering that starts with U. They look to be some sort of unmarked police car. I would venture to say that it's Illinois state police, but the state police have a license plate that says state police on them. Anyone happen to know which agency these car belong to?