Local New Jersey police departments that compile data collected by automated license plate readers store the information for at least five years by order of the state's attorney general. That means thousands, if not millions, of pictures can be stored in a departmental database, the ACLU said in a report issued last week.

But police cannot sort through collected images on a whim, Phillipsburg Chief James Faulborn said. To track a plate, police must have a legitimate reason, such as a vehicle being suspected in connection with a crime, he said.

"As far as using that information, anything garnered from that is strictly governed by attorney general guidelines and case law," Faulborn said, adding there can be serious repercussions should an officer or department stray from the rules.

Police departments around the country are using automated license plate scanners to solve crimes, and in the process, collecting millions of digital records that could be used to track innocent U.S. drivers, the American Civil Liberties Union said.

In a new report, "You Are Being Tracked: How License Plate Readers Are Being Used to Record Americans' Movements," the civil rights group details how the scanners, affixed to police cars, bridges or buildings, capture images of passing or parked vehicles and note their location, uploading that information into police databases. Departments keep the records for weeks or years, sometimes indefinitely.

The ACLU says the scanners assemble a "single, high-resolution image of our lives" and has proposed police departments immediately delete any records of cars not linked to a crime.

"There's just a fundamental question of whether we're going to live in a society where these dragnet surveillance systems become routine," said Catherine Crump, a staff attorney with the ACLU.

In New Jersey, departments in Flemington, Clinton Township and Phillipsburg all have a cruiser equipped with the technology.

New Jersey State Police are currently developing an internal policy regarding the usage of license plate readers, although none of the devices are in use, Sgt. Adam Grossman said.

Flemington police Sgt. Jerry Rotella said in the course of an average 12-hour shift his department's reader will automatically scan between 1,900 and 2,500 license plates. If a plate has been flagged on a "hot list," a siren will sound and the computer indicates the seriousness of the problem, whether a registration is expired, the vehicle has been reported stolen or the owner is wanted on a warrant.

If the plate is clean, a picture of the plate and bumper will be recorded, along with the date and time the photo was taken.

In Jersey City, the city collected more than 2 million plate images on file. The ACLU estimates that it has some 10 million images on file, making it possible for police to plot the movements of most residents depending upon the number and location of the scanners, according to the ACLU.

In the Lehigh Valley, the Easton police department has a plate scanner, but has yet to install it, according to Mayor Sal Panto Jr. In 2011, the city partnered with Bethlehem on a federal Justice Assistance grant and earmarked its $15,798 portion of the grant for the necessary hardware and software, according to a grant application the ACLU obtained through a Freedom of Information Act request.

Panto said the department is training two officers to use the technology that will go live in a few weeks and be used only to scan plates for parking violations.

"If you have $1,000 in parking tickets, it will notify an officer that this person has outstanding violations," he said.

, Pa., police Chief Dan Pancoast said the department is considering purchasing a license plate reader in the near future, and Allentown police Capt. Jorge Medero said the city employs the technology. Medero could not be reached for additional comment about how long Allentown has used the plate readers.

Pennsylvania State Police had 25 license plate scanners during a trial period from 2009 until last year, when the devices were returned to the Pennsylvania Auto Theft Prevention Authority, a spokesman said. Several of the units were used in the Lehigh Valley, mounted on the back of troopers' cars, according to Capt. William Teper of Bethlehem-based Troop M.

"They were very effective and used primarily to locate stolen vehicles or vehicles with a (be on the lookout alert) attached," he said in an email last week, adding there are no immediate department plans to purchase cameras of their own.

The civil rights group says it is not opposed to using the scanner to fight crime, but contends the large volume of collected images leaves citizens open to abuse "for official purposes, like spying on protesters merely because they are exercising their constitutionally protected right to petition the government, or unofficial ones, like tracking an ex-spouse."

The ACLU's study, based on 26,000 pages of responses from 293 police departments and state agencies across the country, found that license plate scanners produced a small fraction of "hits," or alerts to police that a suspicious vehicle has been found.

Critics contend the scanners and resulting databases show little benefit to public safety or law enforcement.

Supporters, however, have declared the scanners a valuable tool for detectives tracking suspected criminals, finding a stolen vehicle or missing child, and issuing motor vehicle summonses. In addition, they say the technology is legal and simply automates a practice that's been done for years.

"There is no expectation of privacy with a license plate to begin with," Rotella said. "This is as much recording as if you were to go to a bank and they record you withdrawing money from account. It's a public place and chances are you are going to be recorded and filmed, and that's all this does."

The Associated Press contributed to this report.