Officer David Callister parks his patrol car under a shady interstate overpass, angling his cameras to target a flurry of passing traffic. Then he waits.

Infrared units mounted to the front of Callister's vehicle scan the license plates of a Casa Grande firefighter, an Ohio State football fan and everyone else who drives past as he hunts for stolen vehicles.

Every plate is photographed, time-stamped, labeled on a GPS map and automatically logged into an Arizona Department of Public Safety database. An electronic voice alerts Callister to stolen vehicles within seconds after they pass, giving him the ability to make quick arrests.

Callister is among the growing number of Arizona officers who use cameras to scan thousands of plates on a daily basis, sweeping parking lots and highways to recover stolen vehicles faster than ever before.

In the past two years, the technology has been lauded as more than a tool to thwart car thieves. DPS claims its program has the potential to intercept violent criminals and Amber Alert suspects, though lawmakers and activists raise questions about invasion of privacy for average citizens whose plates are scanned.

In a state that routinely ranks among the top five in the U.S. in auto theft, DPS scanned more than 1.6 million plates since introducing its first cameras in 2006 - leading directly to 122 felony arrests.

Callister, a DPS Border Crimes Unit officer, uses a set of the agency's 25 plate-reader cameras to track stolen vehicles south of Phoenix. He said the system supplements everyday police work, freeing him from the routine checks that used to consume his time.

"Three years ago, all I had in my car was a radio to talk to a dispatcher, and I had to wait my turn," Callister said. "If I was lucky, I could run 10 vehicles a day," he said. "Now, with the plate reader and my computer, I've had days when I've read over 8,000."

Officers in Valley cities use cameras to recover missing passenger cars. But Callister said stolen trucks seized on freeways are often linked to other border-related crimes. Dodge pickups, F-250s, Escalades and Tahoes are more commonly recovered than the Hondas or Toyotas labeled as the most-stolen vehicles in the U.S.

"That's not what's going to Mexico," Callister said. "Those are the little joy-riding vehicles."

Callister said he uses his cameras to hunt the "baddest of the bad guys" - thieves wanted in connection to the trafficking of drugs, cash and illegal immigrants. Gov. Janet Napolitano recognized him in December for using the cameras to recover 75 stolen vehicles and 140 stolen license plates in a 16-month period.

As Arizona police expand plate-reader technology, officials in California, New York and throughout Europe continue to add systems.

New York, for example, boasts hundreds of cameras on toll roads, bridges and overpasses in addition to those on state troopers' vehicles. One such camera disproved a man's alibi earlier this year, showing his true location the night a family was slaughtered at their home in 2007, leading to his eventual conviction on murder charges.

DPS and other police agencies already have proven that plate-recognition cameras solve crimes. The question is where and how Arizona will expand the technology in the future.

Impact on Arizonans

Experts believe the influx of plate-reader cameras helped reduce Arizona's high rate of auto theft, at least in part, because the systems are becoming a more common aspect of everyday police work.

The arrest of just one auto thief leads to a substantial drop in stolen vehicles because a single thief can steal several cars in a day or week, according to Phoenix police.

Investigators believe a recently adopted Arizona law that requires victims to accurately report auto theft also will continue to help the numbers fall.

Phoenix reported a 26 percent drop in auto theft since 2007. The Arizona Automobile Theft Authority, a state agency funded by insurance companies, reported that statewide thefts dropped 11.9 percent last year alone. Arizona auto-theft rates had been on a slight decline since police reported more than 56,800 vehicles stolen in 2002.

Enrique Cantu, executive director of the Arizona Automobile Theft Authority, said more Arizona cities have applied for grants through the agency with the hope of placing plate readers on local streets.

As local agencies acquire the technology, larger agencies such as DPS will continue to test the next generation of cameras. A wireless plate reader hidden in an orange construction barrel, for instance, allows an officer to monitor traffic going the opposite way from the patrol vehicle.

Victims are often grateful to see their stolen vehicles quickly returned by officers using plate-readers.

Joseph Justice, a north Phoenix resident, said his Chevy Silverado went missing in July from a Paradise Valley Mall parking lot.

"It's a hell of a feeling," the 63-year-old said. "You walk out of the mall and see your truck isn't there. You feel like you've been violated."

Justice reported his truck missing around 10 a.m. By 3 p.m., DPS found it.

Callister recovered the truck on Interstate 10 when it passed his plate reader - along with a Dodge truck that had been stolen from a Goodyear movie-theater parking lot.

Mapping motorists

U.S. plate readers were introduced as an auto-theft deterrent, but investigators talk about using the cameras to create a virtual Arizona crime map, widening the scope beyond stolen vehicles.

By logging the daily location of thousands of registered automobiles, investigators may be able to narrow down the locations of people they are looking for.

The automated technology, for instance, gives officers the ability to check the license plate of each vehicle parked outside a known drug house or note what cars were parked outside a bank before and after a heist.

In October, Callister stopped a motorist on I-10 near Casa Grande for driving too close to another vehicle. The stop led to the discovery of $175,000 in cash and raised suspicions of money laundering.

To search for the man's possible criminal associates, detectives could easily check the list of license plates on vehicles that passed before and after the man's vehicle.

Plate-readers might be a boon for investigators, but agencies such as DPS already have grappled with the possibility of public resistance from those who fear the technology threatens the civil rights of law-abiding citizens. Of the thousands of license plates scanned each day, only a small fraction of the vehicles are tied to some possible criminal activity.

DPS, working with statewide task forces, could emerge as the central agency to store the data from the scans - but the agency has yet to establish guidelines on how to use the data and how long that information would be saved.

"That's where some people might consider it an invasion of privacy," Callister said, but he downplayed the idea, saying the plates are public information seen on public streets.

"This database is just a big pile of plates and GPS locations," he said, adding that the potential to solve crimes outweighs privacy concerns. "If somebody is involved in a bank robbery, or kidnaps a kid, and they do have a plate, they can go back to see the vehicle was at a (specific) location."

Cmdr. Larry Scarber, who oversees the DPS plate-reader program, said information from the cameras is used strictly to prevent crime.

"We have to be very cautious," Scarber said of the records of vehicle locations. "Right now, we haven't gotten rid of anything."

Police leaders met recently with Arizona Attorney General Terry Goddard to discuss how to manage the vast amount of data without treading on motorists' civil liberties or limiting the technology's crime-fighting potential.

Big Brother concerns

A Goddard spokeswoman said discussions about how to regulate DPS plate-reader data are ongoing.

Alessandra Soler Meetze, executive director of the American Civil Liberties Union of Arizona, said she is concerned about how police technology could outpace legal standards.

"The problem is we really have no reassurance it's going to be focused on the bad guys," Soler Meetze said.

Arizona legislators have provided little guidance on how to regulate the technology since Mesa police pioneered Arizona's first plate-readers in 2005.

State Sen. Pamela Gorman, R-Anthem, who did not return calls for comment, introduced a bill in 2007 that suggested DPS should dispose of license-plate images within 24 hours unless the data is tied to an ongoing investigation. The bill went nowhere.

State Rep. Jerry Weiers, R-Glendale, has introduced legislation to help fight license-plate fraud and is a supporter of plate scanners. He said he hears more opposition to automated enforcement technology from legislators seeking to regulate government surveillance than from constituents who want their stolen cars back.

"Big Brother - it's one more thing where people are concerned that the government is looking at you," Weiers said.