Illinois state officials have given national rental car companies official notice that immigration enforcement agents using their vehicles are not allowed to swap the rental’s assigned license plates for other plates to disguise the vehicles, and if they do, the rental car companies could be held liable.

According to documents obtained by NBC News via the Freedom of Information Act, the Illinois secretary of state’s office sent letters to at least 19 national car rental headquarters stating that they had received public complaints of immigration agents switching license plates on rented vehicles when Operation Midway Blitz, an extensive government deportation operation, was active in the Chicago area.

The letters were sent to Alamo, Enterprise, Budget, Hertz, Ace and other vehicle rental companies. They did not respond to requests for comment.