A semi-secret surveillance program developed by the Drug Enforcement Administration is collecting location information about "millions" of Americans through the use of a license plate-reading system to which state police departments also contribute data.

It's not clear how large the program is, but according to documents obtained by the American Civil Liberties Union through a Freedom of Information Act request, at least 100 license plate readers across the United States are part of the DEA-run network.

The program was started to assist the controversial civil asset forfeiture program which allows law enforcement to seize a suspected criminal's property without the suspect ever being convicted of the alleged crime. (More on that in this excellent New Yorker report.)

Now the program is being used to assist in the hunt for suspected rapists, murderers, and kidnappers, too. The Wall Street Journal reports that any department cleared by the El Paso Intelligence Center can query this database of nationwide location information.

The result is a national surveillance program with an unknown number of contributors offering up location data about millions of Americans; all to a database used by an untold number of police departments without any public oversight regarding their searches.

That's a problem. Backchannel reported in December that police have used their access to license plate readers to stalk former colleagues, and IB Times revealed earlier this month that Gov. Chris Christie (R-NJ) used location data to smear a political rival.

The ACLU said in a blog post that it plans to reveal more information gleaned from the documents handed over as the result of its FOIA request over the coming weeks.