Police in South St. Paul, West St. Paul and Inver Grove Heights have joined a rising number of police departments using automatic license-plate readers to find stolen vehicles.

Using a grant from the Minnesota Department of Commerce, the departments each purchased one reader, which ranges in cost from $15,000 to $20,000, South St. Paul Police Chief Bill Messerich said.

West St. Paul Chief Bud Shaver also expects the reader to be useful in finding drivers with suspended or expired licenses.

The readers are mounted on the roof of a squad car on the bar that holds the emergency lights. The camera-like technology automatically reads license plates as the officers drive, and if a stolen vehicle is detected, it alerts the officer. It also displays expired plates or license revocations.

The readers record the time, date and location of every car they detect. Privacy advocates have called for standards to govern the use of such data. South St. Paul will delete the information after two weeks, Messerich said.