A California man charged recently with kidnapping and assaulting a 12-year-old girl was caught, in part, by license-plate recognition software -- software that some privacy activists would ban.

According to news accounts, Donald Bachmann stole a pickup truck in San Jose, ditched it at a 7-Eleven, and then stole a white Toyota sedan. The police were notified.

Shortly thereafter, the 12-year-old girl was walking home with her 9-year-old sister. Police say Bachmann rammed the Toyota into the older girl intentionally, severely injuring her. Her little sister rushed to aid her, but Bachmann allegedly kidnapped the bleeding girl and drove away. The 9-year-old immediately reported the crime.

Less than 15 minutes later, police say Bachmann parked the car, punched his victim and tried to sexually assault her. Miraculously, she escaped and ran to a nearby home. The residents called the police. Bachmann drove a few miles and ditched the Toyota.

The police responded to the 7-Eleven, collected evidence from the stolen truck, and found Bachmann's fingerprints inside.

Hours later, Officer Max Boyer was on patrol, with license-plate recognition technology mounted on his patrol car. It can scan the license plate of every vehicle it passes. The computer compares the plates to databases of stolen vehicles. Boyer drove by the Toyota and a computer voice in his car said, "Stolen car." He stopped to investigate. Police found blood in the Toyota belonging to the 12-year-old girl.

Bachmann was arrested, and charges have been filed.

The software used to crack this case opens a new chapter in the ongoing debate between crime-fighting technology and privacy.

The Fourth Amendment provides that we have the right to be secure in our "persons, houses, papers, and effects, against unreasonable searches and seizures." The question is, how much privacy are we as citizens willing to give up to allow law enforcement to solve crimes?

The limits of reasonable searches have been tested countless times in court. In 2001, for example, the Supreme Court held in Kyllo v. United States that law enforcement couldn't use a thermal imaging device from a public street to detect relative amounts of heat emitting from a private home. The Court found the search of the home using the thermal imaging device was an unreasonable intrusion and expressed concern about permitting "police technology to erode the privacy guaranteed by the Fourth Amendment."

So, what about license plate recognition technology? It has been used for more than 20 years in Britain and Europe, but only recently added to the American law enforcement toolkit. The answer to the question of whether this technology is constitutionally unreasonable may be found in other Fourth Amendment cases, what the technology does, and how it is used.

The police can fly over your home and look down on your property without your permission. They can sit in their cars on a public street and monitor any activity they want using the naked eye or binoculars. And, more to the point, they can drive down a public street and manually enter license plates into their computer to check if they're stolen. License-plate recognition software allows an officer to check up to 12,000 plates per shift, compared to about 50 per shift if done manually. In contrast, the Supreme Court struck down the use of thermal imaging devices in Kyllo, in part, because the devices could "read" thermal radiation that the human senses couldn't detect from public streets.

Today, license-plate recognition software is used in airport parking lots, as access control monitors to restricted areas, in tollbooths on toll roads, at border-control areas, for traffic enforcement at intersections, and to assist city administrators in controlling municipal traffic loads.

Law enforcement needs the best technology available to fight crime. In the California case, license-plate recognition software allowed the police to find evidence in the getaway car they might not have ever found. Its use was not an unreasonable intrusion into anyone's privacy. It only ensured the officer didn't miss something that was in plain view. As the Supreme Court said in a similar context in 1886, "the eye cannot ... be guilty of trespass."

The benefits of this technology are obvious. Privacy proponents say we're approaching the point where the government will be able to monitor and record most activities we engage in. Fine, let the debate continue. But can anyone reasonably argue that the benefits to society this technology offer are outweighed by a "loss of privacy"?

Cully Stimson, a former prosecutor and defense attorney, is a senior legal fellow.