According to the Wall Street Journal, the surveillance programme was initiated by Drug Enforcement Administration (DEA) in 2008 in states near the US-Mexico border, such as Arizona, California, Nevada, New Mexico and Texas.
At the first the aim was to track cars in that region and, hopefully, staunch the flow of drugs and drug money into the USA. Since then the programme has grown to encompass states as far away as Florida and New Jersey.
The newspaper wasn't able to obtain a full list of participating jurisdictions, but some states, such as Utah, have refused to be a part of the DEA's scheme.
Documents obtained by the American Civil Liberties Union also indicate that one of the DEA's primary goals is asset forfeiture, a US legal mechanism that allows law enforcement agencies to seize cash, cars, guns and drugs without the need for a conviction.
Aside from the information garnered by the Journal from former government employees and documents acquired via freedom of information requests, much remains unknown about this surveillance system.
We do know, though, that the system relies primarily on cameras placed along the interstate highway system, as well as licence plate readers operated by participating federal, state and local governments. All of the data gathered from these systems is then fed into a central server run by the El Paso Intelligence Center (EPIC) in Texas.
Accredited officials from participating agencies can then search the EPIC database for the movement and whereabouts of any licence plate in the system.