In coming days, as thousands of cars travel Interstate 494 and park at the Mall of America, Southtown and other shopping areas, they may unknowingly get the new eye of Bloomington police.

It's a virtual patrol partner mounted on two squad cars that works silently until there's something worth saying. Then an alarm sounds, and an image pops up on a computer screen for the police officer to look at.

Cameras mounted on these cars can scan thousands of license plates on passing and parked vehicles in a single work shift, sounding an alert when a plate that's on a state database of stolen or wanted vehicles is located. Police in Lakeville, Minneapolis and St. Paul and the State Patrol use similar camera systems, all of them added in the last couple of years.

Bloomington's two cars were equipped in March after the city received a $50,000 auto theft prevention grant from the state. Seventeen stolen cars have been recovered since then. The system also has nabbed wanted people in stolen cars who had felony warrants, said Bloomington police Sgt. Mark Elliott.

"We're pretty happy with it," he said.

Tailor-made, speedy results

Camera systems can be tweaked to meet the individual needs of departments.

In Bloomington, with its freeways and the massive mall parking lots and ramps, each police car has four cameras. Two face forward, snapping photos of oncoming and passing vehicles and close-ups of their license plates. Two more cameras face out from either side of the squad, so police can cruise between two lines of parked cars shooting images of plates on both sides.

In the car, a small computer equipped with optical recognition software reads license plate numbers and letters, and checks them against the database of wanted vehicles.

The speed of the digital reading is a huge advance for police, Elliott said. Before, officers had no way of knowing if they were driving past a stolen car. They would interrupt whatever they were doing to write down or hand-enter the license plate of a suspicious vehicle into their computers and wait for a verbal or computer response.

Now, with a scanning system that's 90 to 95 percent accurate, they can eyeball the images of a flagged vehicle to see if it matches those on a list of wanted vehicles that are updated twice daily from the state. The scanner can read licenses when both the squad car and target vehicle are moving up to 80 miles per hour, and will read licenses on parked cars when the squad car is moving up to 50 mph.

Spotting trouble on the fly

Consider the Bloomington officer who earlier this year responded to a medical emergency at an apartment.

"She was thinking about where she needed to go ... and what first aid might be needed, and as she entered the parking lot she passed a stolen car and the system alerted," Elliott said. "She called other officers. They could recover the car while she went on to take care of the medical."

St. Paul has several cars equipped with the license plate readers, while the State Patrol has one car with the system patrolling in St. Paul. The patrol's Capt. Matt Langer said that troopers look for drivers with suspended and revoked licenses or canceled registrations.

"Those drivers have been proven to be more involved in serious crashes," he said. The system has added enforcement benefit, Langer said, because those drivers often have a history of not paying fines or child support.

How it works in Lakeville

Lakeville is thought to have the longest history with license plate readers. Police Chief Tom Vonhof said a car was equipped with the system in 2007 after the city got a grant from Target Corp. That system reads up to 2,000 plates during a 12-hour shift.

A lot of drivers with suspended and revoked licenses or who had warrants have been caught since then, Vonhof said. It has also been used to look for vehicles linked to Amber Alerts or domestic violence cases.

He said its value is greatest where communities are densely populated and traffic is thick. The Lakeville car that has the reader now needs to be replaced, and a decision will have to be made whether to keep the scanner in a city that sprawls 38 square miles. Vonhof said he is leaning toward keeping it.

Few beefs, but not perfect

Departments with license readers say they've heard few accusations that they're acting like Big Brother. Images of vehicles are kept for a time -- the State Patrol keeps them for two weeks -- and then they're dumped. Elliott said the computer does not run criminal history checks, and all reading and recording of plates happens in public places.

Though the scanning system is speedy, it's not faultless. Dirt and road salt can obscure plates. Elliott said Bloomington's system can't distinguish between plates from different states. And sometimes it tries to read things like "no parking" signs, which means the officer in the car still wields the ultimate judgment on what information is useful and what's not.

Mary Jane Smetanka • 612-673-7380