The American Civil Liberties Union sent dozens of coordinated letters to federal and local police agencies across the country requesting information about how they use license plate readers, real-time tracking devices that have little public oversight.

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.

But 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.

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, the devices both mounted on police cars and on stationary poles. Last year, the city had the highest concentration of them in the nation.

The ACLU filed federal Freedom of Information Act requests with the Departments of justice, homeland security and transportation to learn how the federal government uses tag readers and funds the programs. Additionally, 38 local ACLU chapters — including ones in the District, Maryland and Virginia — sent similar requests to state and local law enforcement agencies.

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.

“We’re trying to find out how widespread this practice is and what police are doing in terms of data retention,” said Rebecca Glenberg, Legal Director of ACLU Virginia. “We want to ensure departments have clear policies regarding the use and retention of data that do not result in widespread surveillance of the population at large.”

The plate readers are different from red light or speed cameras, which issue traffic tickets and are tools for deterrence and enforcement. The readers are an investigative tool, capturing a picture of every license plate that passes by and instantly analyzing them against a database filled with cars wanted by police.