The American Civil Liberties Union filed a federal lawsuit Tuesday against the departments of Justice and Homeland Security seeking information about license plate readers, real-time tracking devices that have little public oversight.

The ACLU contends that the agencies have not responded to freedom of information requests the ACLU sent in July, apart from acknowledging them. The requests sought information on the automated cameras, including how they are funded and used to track cars. A response was required within 20 working days.

The lawsuit was filed in Massachusetts, but has national and local implications in the Washington region.

Locally, hundreds of cameras on public roads in the District and its suburbs scan license plates as cars drive by, helping police pinpoint stolen cars and fleeing criminals. The cameras can capture as many as 1,800 images of license plates a minute.

The program quietly has expanded in recent years as police agencies have begun storing the information from the cameras, building databases that document the travels of millions of vehicles.

The cameras “raise the prospect of pervasive and prolonged surveillance of innocent Americans’ movements and start to pose a serious threat to Americans’ privacy,” reads the ACLU’s complaint. “Yet there has been little public disclosure of the policies and procedures that are in place to protect individual privacy.”

The Department of Homeland Security declined to comment on the lawsuit citing pending litigation, and The Washington Post is awaiting a response from the Department of Justice.

Each police department in the D.C. region determines on its own how long to retain its data. The District has more than one plate reader per square mile, mounted on police cars and on stationary poles. Last year, the city had the highest concentration of them in the nation.

In addition to the information requests from the federal agencies, 38 local ACLU chapters — including ones in the District, Maryland and Virginia — sent requests to state and local law enforcement agencies asking how they use tag readers and fund the programs. The ACLU also sent a request to the Department of Transportation, and is working with that agency to get information.

Police departments are grappling with how long to store the information and how to balance privacy concerns against the value the data provide to investigators.