Nearly six years ago, two federal law enforcement agencies considered using license plate readers (LPRs) at gun shows—at least in the Phoenix, Arizona area.

LPRs scan plates at a very high speed—often 60 plates per second—and record the date, time, and precise location that a given plate was seen. Typically, on a patrol car, that plate is then immediately compared to a list of wanted or stolen cars, and if a match is found, the software alerts the officer. But all scans are routinely kept by various law enforcement agencies for long periods of time, sometimes as long as years or more.

According to a heavily redacted set of documents that the American Civil Liberties Union (ACLU) received and published earlier this week, a 2009 e-mail presumably from the Drug Enforcement Agency states that the “DEA Phoenix Division Office is working closely with the [Bureau of Alcohol Tobacco and Firearms] on attacking the guns going to [REDACTED] and the gun shows, to include programs/operation with LPRs at the gun shows.”

DEA Administrator Michele Leonhart said in a released statement, “The proposal in the e-mail was only a suggestion. It was never authorized by DEA, and the idea under discussion in the e-mail was never launched.”

The Department of Justice, the parent agency of the DEA, has declined to respond to Ars’ specific questions about its use of LPRs.

“The DEA’s statement alleviates some concerns, but if the program was cancelled, why didn’t we get any documents reflecting that decision in response to our FOIA request?” the ACLU wrote in a blog post. “The agency should now release such documents, and also create and release a written policy that it will not target First Amendment-protected activity in the future.”

A 2011 e-mail from the same set of documents says that the DEA's LPR program can "store up to 10 photos per transaction including 4 occupant photos."

Law enforcement has argued as recently as last week that such records are indispensable for recovering stolen cars and for using in criminal investigations. The devices are rapidly being deployed by state and local law enforcement agencies—notably some communities (such as the wealthy small town of Piedmont, California) have even put them up as an electronic border against their neighboring towns.

However, privacy advocates counter that the devices are being used at all levels of government to gather bulk data on American drivers’ whereabouts for no reasonable criminal justice purpose.