Long Island law enforcement officials are increasingly using license plate readers — once just a device to go after unregistered drivers and traffic scofflaws — as an intelligence tool, mining the data for crime trends and using it to solve homicides and robberies, police said.

The Nassau and Suffolk County police departments, as well as police in villages such as Freeport, use an expansive network of more than 100 license plate reader cameras, some visible to the public and others intentionally obscured.

The license plate readers capture the image of millions of license plates and vehicles each year across the Island and have emerged as the pre-eminent tool for police solving pattern crimes, such as burglaries and bank robberies, officials said. Other police departments such as Hempstead, Glen Cove, Port Washington, Kings Point, Long Beach and Lynbrook have license plate readers.

Nassau Det. Sgt. Patrick Ryder, commanding officer of the department’s Asset Forfeiture and Intelligence Unit, said the department, which has about 50 cameras, moves them to different locations based on crime trends and then carefully examines the data for clues about a potential suspect.

“It is the No. 1 tool that we use here,” Ryder said. “If I could put them on every block in Nassau County, I would. We data mine through it, to try to figure out: Who’s the anomaly here? Who’s the outlier that’s entering our area of crime? Do you live here? Are you in the time of our pattern? Could you be an outside resident, a visitor or an employee? Or could you be a bad guy?”

License plate readers have been used by local police departments for several years — many fixed on police vehicles — to root out traffic ticket dodgers and unregistered cars, but advances in the technology have transformed the once-grainy pictures to sharp digital images.

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In several recent cases cited by police, a witness has had only scant details; for example, telling police a green car was seen leaving the scene of a homicide. Investigators then scoured the license plate reader footage in that time period and location and made a break in the case, police said.

Looking to solve a pattern of about 20 burglaries in New Hyde Park a few years ago, Ryder said, police set up covert license plate readers in the area and analyzed the data and made an arrest.

“We were able to take 20,000 plates down to four people that did not belong there,” Ryder said. “And one of the four people, when we checked the four people, he was a burglar. He was from Queens and had a history of burglaries.”

The Suffolk County Police Department has about 30 cameras — the vast majority affixed to marked patrol cars, Chief of Department Stewart Cameron said.

The department got its first cameras in 2006 and they were primarily used for traffic enforcement, Cameron said. But with technology advancements, “They’ve become a useful investigative tool as well,” he said.

Police agencies across the region — including the county, village and state police, as well as federal agencies — share the data, officials said. And police say they’d like to buy more license plate readers. Each camera costs about $30,000.

They don’t come without controversy. The American Civil Liberties Union in 2013 issued a report raising concerns about their usage, citing, for example, the NYPD’s use of license plate readers on unmarked police cars outside of mosques in order to record attendees.

“Increasingly, they are capturing drivers’ locations outside church, the doctor’s office, and school, giving law enforcement and private companies that run the largest databases the ability to build detailed pictures of our lives,” the report said. “Location data can reveal extremely sensitive information about who we are and what we do.”

Joseph Giacalone, a retired NYPD sergeant and adjunct professor at John Jay College of Criminal Justice, said license plate readers capture the plates and vehicles, which are publicly visible and offer no expectation of privacy.

“The ACLU is fighting this, but meanwhile they want to have like all the cops wear body cams and now everyone will be on video,” Giacalone said. “So you can’t pick and choose your battles. And when you’re out in public, you have no right to privacy anyway. You don’t have an absolute right to privacy in your car. Licenses and registrations are not rights, they’re privileges and they can be revoked at any time.”

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Long Island police officials said they have access rules to prevent abuses. Nassau keeps its license plate reader data for five years, as recommended by a state panel, officials said.

“We go to great lengths to maintain the privacy of people when it comes to cameras and everything else,” said Ryder, who said the system is audited and that access to the system is restricted to certain police personnel. “People think that we sit there and stare at cameras all day watching the video. We don’t. We watch them when the crime occurs. The reason the video’s there is to capture that forensic evidence. . . . It’s capturing potential forensic evidence that will be used down the road, but in the meantime, it captures it, it stores it and we only look at it when we need to look at it for a crime or a certain plate already captured.”

Officials used license plate reader, or LPR, data to solve a rash of smash-and-grab commercial robberies last year. Three Brooklyn men were eventually arrested and charged in connection with stealing cash and more than $200,000 worth of jewelry at nine Nassau businesses, police said.

The break in the case, which was investigated by Nassau police and the district attorney’s office, came after cops pulled the LPR data after a Massapequa jewelry store burglary.

“We went to the plate reader data . . . we captured that plate, tied it to the car, which tied it to the girlfriend of the bad guy,” Ryder said. “The DA’s squad was on the car the next day. Two days later they ended up arresting him doing another burglary and closed the case.”

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Nassau County District Attorney Madeline Singas, in a statement Saturday, called license plate readers “an emerging addition to a broad set of investigative tools used to solve crimes; they are most effective with the combination of proper targeting and good traditional investigative practices.”

The city of London famously installed a “Ring of Steel” — including surveillance cameras and license plate readers — in the 1990s, intended both as a counterterrorism measure and an intended foil for common criminals. The system was replicated about a decade ago in lower Manhattan.

Freeport, New York’s second-largest village with a population of about 50,000 in 4.3 square miles, has erected its own “Ring of Steel,” putting the license plate readers at every entrance to the village, investing about $750,000 for the system.

Since early November, when its 34 license plate readers were installed — perched on light poles around the village — Freeport police have used the technology to catch an alleged killer, a trio of robbery suspects and a church burglar.

Freeport resident David Spencer, 37, of High Place, had burglarized churches in Baldwin, Freeport and Roosevelt by breaking a window and swiping “Yamaha keyboards, cash, a mountain bike, assorted jewelry and ceremonial chalices,” police said.

And in January, Freeport police spotted a 2001 Chevy Tahoe that was reported stolen entering the village on a license plate reader. The driver, Tremain Williams, 36, was arrested as a suspect in a Virginia homicide and the vehicle he stole belonged to the murder victim. In the Tahoe, Freeport cops found a loaded M4 assault rifle, police said.

For the homicide arrest, officials said, the village spent $5,000 in police overtime.

Freeport police have gotten so many hits for expired car registrations that they can’t keep up with the volume. Freeport Mayor Robert Kennedy said with increased overtime expenditures due to the system, he wants to hire another 10 officers.

“We’re really becoming overwhelmed with doing the job for the entire Nassau County because all these cars are coming through — we’re arresting them,” Kennedy said. “So I’m putting men on overtime every day, every day, every day, for all these stolen vehicles, stolen plates, hit-and-runs that we’re picking up that nobody would have ever picked up before.”

Since Freeport began using its license plate cameras on Nov. 2, police have made 25 arrests, impounded 328 vehicles, and issued 1,290 summonses overall, officials said. The department has also assisted other agencies with 15 significant investigations. Overall, the cameras have scanned 14.6 million license plates, officials said.

Of the 25 arrests from Nov. 2 to late January, 15 were for stolen vehicles. In the same time period the previous year, the department made only one stolen car arrest, according to department statistics.

Freeport Police Chief Miguel Bermudez said the village’s system, run with fiber optic cable powered by the village’s power plant, is impenetrable.

“We’re looking at every major and secondary road coming into Freeport and it’s giving us that information,” Bermudez said. “We don’t have to have a police officer assigned to that location. We’re gathering that data strictly through this technology. And then from there the officers are free to utilize that information, including our operators in the room, also our crime analysts, to help us, and then that information is passed on to our detectives or our officers for investigation.”

Bermudez said the department actively monitors the system about five hours a day, looking at the previous 24 hours of data to look for patterns.

“This stolen car’s been in town three times in five days,” Bermudez said. “That’s what we’re looking at.”

Giacalone said the license plate readers, especially the way Freeport has deployed them, have a huge deterrent factor.

“That million bucks that they spent could be paid back 10 times more if they don’t have to hire extra police officers and the pension benefits and everything else that goes with it.”

Kennedy, the Freeport mayor, said: “I want the criminals to know: Don’t come to the Village of Freeport. You come into Freeport and you commit a crime, we’re gonna get you.”