Virginia Attorney General Ken Cuccinelli (R) has issued an opinion that limits the use of police license-plate cameras, a decision likely to force departments across the state to restrict how they track cars on Virginia’s roads.

The cameras, which are affixed to cruisers and roadside poles, photograph license plates of passing cars and are increasingly used by law-enforcement agencies nationwide. At issue in Virginia is under what circumstances the photos violate the privacy rights of drivers whose movements are captured and recorded.

Cuccinelli wrote that when there is an immediate threat to public safety, police can use the license-plate readers to look for a car.

But “passive” collection of the data, such as keeping the cameras on during routine patrol, archiving every license plate along the way, is not lawful, he said.

Police departments in Northern Virginia said they are working to determine whether and how the opinion will change the way they collect and store the license-plate data.

“We are in the process of evaluating the opinion and how it might affect the policies and procedures regarding our use of automatic license-plate readers,” said Fairfax County police spokesman Don Gotthardt. “As of now, no decision has been made. Therefore, our procedures will not change for the time being.”

The plate readers, which snap 1,800 photos a minute, are different from red-light or speed cameras. They are an investigative tool, capturing a picture of every license plate that passes by and instantly checking them against a database filled with cars wanted by police.

The pictures are analyzed in real time and stored with a date, time and location. In some jurisdictions, they are kept in a database indefinitely, whether or not they are used in criminal investigations.

With them, police have quickly been building archives that can track a driver’s movements.

Many police agencies currently use the readers in this “passive” way as officers cruise around on routine patrol.

Cuccinelli wrote that in order to use the cameras, the need for them must be “clearly established in advance,” or else it violates the state’s data privacy statute. He also said that data cannot be retained unless it has been analyzed and found to be “criminal intelligence information.”

His opinion, issued last month, was in response to a request for legal guidance by the superintendent of the Virginia State Police, Col. W.S. Flaherty.

A state police spokesman said in a statement that the department abides by the opinion and uses the cameras “to target specific vehicles” in cases including child abductions and car theft.

While many police departments across the region — and the country — use the cameras, most are regulated by each agency’s internal policies. Until now in Virginia, each department came up with its own rules. That is also the case in the District, which has the highest concentration of them in the country, and in Maryland.

But officials and lawmakers nationwide have begun to weigh in on when it is appropriate to use the cameras and when it is an intrusion of privacy. In Minnesota, for example, lawmakers introduced legislation that would require police to keep a public log of where and when they collect license-plate data.

In Montgomery County, Rockville City Council member Tom Moore said the mayor and council need to discuss how to handle the information they collect. Currently, the city sends license-plate data to Montgomery County, where it is easily accessible to investigators for a year before it is archived.

“There is a larger issue of how much do we want government tracking the whereabouts of everyone everywhere they go,” Moore said. “The only way to be sure the data is not misused is to be sure it doesn’t exist.”