A license plate reader is seen mounted on the trunk of a Metropolotian Police Department car in Washington. It records license plates and rapidly checks a database of stolen or wanted cars. In Northern Virginia, police have ignored an attorney general’s opinion regarding the images that are collected. (Paul J. Richards/AFP/Getty Images)

Every day, police officers across Virginia scan the license plates of tens of thousands of vehicles and enter that information into their departments’ databases, hoping that it might help solve a crime.

But last year, then-Attorney General Ken Cuccinelli II issued a clear opinion about the use of license plate readers, which have become common across the country: If the information isn’t directly related to a criminal case, it “may not lawfully be collected.”

Virginia State Police quickly adopted a policy to purge the data within 24 hours, unless a crime was involved. But in Northern Virginia, police departments have decided to ignore Cuccinelli’s opinion. They maintain databases of the information — photos with the vehicle’s location at the time — and share them with other police departments in the Washington region.

Cuccinelli’s opinion was not legally binding, police officials in each jurisdiction say, adding that they consulted their city or county attorneys and their prosecutors before continuing to collect the data.

Police in Alexandria retain license plate data for two years, officials said. Fairfax and Loudoun counties keep the information for one year; for Arlington and Prince William counties, it’s six months. In Montgomery and Prince George’s counties, police keep their license plate data for a year. Statewide, some agencies are removing the data immediately, but for others, the length of time is 30 to 60 days or longer, according to Dana Schrad, executive director of the Virginia Association of Chiefs of Police. (District police did not respond to a request for information.)

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The practice has been questioned by some local activists and advocacy organizations, most notably the American Civil Liberties Union, which argues that gathering the information intrudes on the privacy of innocent drivers. Police officials said they recognized the concerns and tried to find a balance between their need to investigate crimes and the privacy of citizens.

Prince William Police Chief Stephan Hudson said that detectives need the data to solve serious crimes or see crime patterns which may not be immediately reported and that they might benefit from a look at which vehicles were at a crime scene at the time of the offense.

“It’s impossible for us to say within a set period of time,” Hudson said, “whether the [license plate] data would be relevant to a criminal investigation.”

Civil rights advocates are unmoved. “They can argue that going door-to-door searching houses without a warrant would help law enforcement solve crimes,” said Claire Gastañaga, executive director of the ACLU in Virginia, “as would listening in on all phone calls. . . . Bottom line: We believe the warrant requirement protects liberty while allowing appropriate law enforcement action.”

The issue is attracting attention in the Virginia General Assembly. Del. Richard L. Anderson (R-Prince William) said he was “considering legislation to codify last year’s attorney general opinion.” He said he had been discussing license plate readers and privacy with his constituents and that “significant citizen concern exists over increasingly pervasive intrusion by government into private and personal matters,” such as NSA data collection and the use of drones.

“Over time,” Anderson said, “government agencies have demonstrated an inability to protect private data from public disclosure, which adversely impacts public confidence in government.”

Cuccinelli said he was “sorely disappointed” that Virginia police departments were ignoring his opinion. “When the people charged with enforcing the law think that breaking it is okay, I’m appalled.”

He argued that “information they gather that way is subject to exclusion [from court proceedings] because it’s illegal.” He said he was considering litigation against the NSA, but added, “Maybe I should look at suing in my own back yard.”

Police cars equipped with cameras can photograph hundreds of license plates a minute, even while moving at high speeds, and they cruise the Washington area daily, compiling searchable databases that privacy advocates say can be used to track an individual’s movements or launch improper investigations. A 2012 study by the Police Executive Research Forum found that almost three-quarters of police agencies nationwide use the readers and that 85 percent plan to use them in the next five years.

Virginia’s law defining government data collection, often called “the Data Act,” begins with a section warning of the dangers of misusing personal information in the data age, so Cuccinelli’s interpretation of the law did not make any large philosophical leaps.

“An individual’s privacy is directly affected by the extensive collection, maintenance, use and dissemination of personal information,” the law starts. “In order to preserve the rights guaranteed a citizen in a free society, legislation is necessary to establish procedures to govern information systems containing records on individuals.”

The key section soon follows: “Information shall not be collected unless the need for it has been clearly established in advance.”

The plate readers are connected to in-car computers that check each license plate against national and regional “hot lists” of stolen cars and missing and wanted people and then sound an alert when one is found. Police in the Washington area began using them in 2004, mainly with federal grant money, because each one costs about $20,000.

Law enforcement officials say that the practice has become an essential tool in developing leads in terrorism investigations and criminal cases and that Cuccinelli’s opinion ties their hands.

“If we were limited by the attorney general’s opinion in the future, the cost-benefit analysis wouldn’t be worth the investment,” said Arlington Police Chief Douglas Scott. “To simply use it only for a stolen-auto hit . . . kind of defeats the investigative purpose and opportunity to have something like that.”

Fairfax and other agencies noted that only law enforcement personnel can access the database, but each agency has a different policy on exactly who within their agency can do so. Fairfax said only a small number of employees can access the license data, and only if the requests from detectives are approved by the chain of command, Chief Edwin Roessler said. In Loudoun, detectives and a handful of officers in specialty units are authorized to access the database, Sheriff Michael Chapman said, and their access is monitored by systems administrators.

In Alexandria, only members of the computer unit can access the database, spokeswoman Crystal Nosal said. In Prince William, “just a couple dozen” people are authorized to check license plate data, Hudson said. In Arlington, Scott said, about 50 officers and detectives are authorized to access the database.

In 2012, many police departments in the Washington area signed a memorandum of understanding to share their databases with each other. It does not create a central database, and the information is available only as long as they are kept by the individual agencies.