WASHINGTON—A federal agent proposed using license-plate readers to scan vehicles around gun shows in order to aid gun-trafficking investigations, according to an internal Justice Department email.

Justice Department officials said Tuesday that the 2009 proposal was rejected by superiors and never implemented. The email was part of a series of Drug Enforcement Administration documents describing how the agency is building a national database tracking the movements of vehicles in the U.S. The documents were obtained by the American Civil Liberties Union through a Freedom of Information Act request and reviewed by The Wall Street Journal.

“The proposal in the email was only a suggestion. It was never authorized by DEA, and the idea under discussion in the email was never launched,’’ according to DEA administrator Michele Leonhart.

National Rifle Association spokesman Andrew Arulanandam said the NRA is “looking into this to see if gun owners were improperly targeted, and has no further comment until we have all the facts.”

The proposal shows the challenges and risks facing the U.S. as it looks to new, potentially intrusive surveillance technology to help stop criminals. Many of the government’s recent efforts have scooped up data from innocent Americans, as well as those suspected of crimes, creating records that lawmakers and others say raise privacy concerns.