LOS ANGELES >> A superior court judge blocked the release of a law enforcement database containing millions of Angelenos’ license plates this week as the information could allow stalkers and criminals to track victims and police patrols, according to his ruling.

In a decision obtained Thursday, Judge James Chalfant agreed with the Los Angeles Police Department and Los Angeles County Sheriff’s Department’s refusal to supply a week’s worth of automated license plate reader (ALPR) data to the American Civil Liberties Union and Electronic Frontier Foundation. Those groups sued in 2012 when the police denied their public records requests.

“The ALPR data contains hot list comparisons the disclosure of which would greatly harm a criminal investigation,” Chalfant wrote, referencing a “hot list” of criminal vehicles the ALPR checks results against. “It also would reveal patrol patterns which would compromise ongoing investigations, and even fixed point data could undermine investigations. Disclosure could also be used by a criminal to find and harm a third party.”

An ALPR uses a specialized camera to capture an infrared image of a license plate, converts it to text and then checks it against a list of wanted vehicles, and audibly alerts an officer if a match is found. The license plate, the date, time and location is uploaded to a server where it remains for two years. The sheriff’s department wants to keep the information “indefinitely” but cannot because of storage limitations, according to court documents.

Chalfant considered the data exempt as “records of investigation” and compared them to video recorded by an undercover officer watching a street corner in a drug sting.

“Frankly the argument that the city is making and the court has endorsed is a very troubling one, that the drivers in Los Angeles are under investigation constantly all the time,” said Peter Bibring, a senior attorney for the ACLU. “It suggests that police can embark on other programs to gain massive amounts of data about civilians that are not suspected of crimes and that data can be protected from disclosure.”

The ACLU and EFF requested the data to ascertain what areas and groups police might target with the technology, to see if the program violates people’s constitutional rights. If the city and county equipped enough cars and street corners with the technology, they could create essentially map the movement of every car in Los Angeles at once.

“It’s our view that by equipping vehicles with license plate readers, police are collecting as much information about Los Angeles drivers as they can because license plate readers target any cars that come in the car’s field of view. They’re not doing it in a targeted fashion, they’re doing it indiscriminately,” Bibring said.

“(The California Public Records Act) is meant to prevent Al Capone from asking the police for his file, so he knows where he’s in trouble, where he’s not and who has information on him. It is not meant to prevent the public understanding police operations outside the context of specific investigations. We think that’s what’s happening here.”

“If the underlying data isn’t public, it’s difficult to understand how severe the privacy violation is,” he added.

The LAPD and LASD do seem to target areas with high Hispanic or African American populations. The LAPD placed 32 fixed license plate readers in Southeast L.A. and the Hollenbeck (East L.A.) area, according to court documents. Six of the 17 LASD operated in 2008 were in Compton, while four existed in La Habra Heights at that time, Chalfant noted. LAPD told the court they operate 242 cruisers with the ALPR technology, but LASD did not disclose its current numbers. Neither provided information about where those cruisers patrol.

The two agencies reviewed a random week and found they collected approximately 3 million license plates in that period. The LAPD and LASD share their database with more than 20 other agencies. They said they’ve used the database to solve robberies and homicides.

The ACLU argued that the agencies could provide a redacted version of their database to protect people’s personal information, but Chalfant wrote that the redaction would not stop a criminal from tracking police.

He agreed the information could potentially help the EFF and ACLU determine if the LAPD and LASD abuse the system, but that the damage from such information would outweigh any benefit to public interest.