WASHINGTON (AP) — The American Civil Liberties Union says in a new report that local police departments are amassing millions of digital records on the location and movement of Americans using automated license plate scanners.

The report, released Wednesday, found that there are tens of thousands of license plate scanners scattered across the United States. The scanners are affixed to police cars, bridges or buildings and capture images of passing or parked vehicles. The police dump that information into databases that can be reviewed weeks, or even years later.

Law enforcement officials say the practice is legal and helps to automate work the police were already doing. The ACLU says stricter rules are needed, such as immediately deleting records not tied to an investigation.