Both Flock Safety and Ring have worked with law enforcement, and the latter's relationships with hundreds of police departments across the country, in particular, has drawn scrutiny from civil liberties groups and others. Advances in surveillance technology have spurred an ongoing debate over privacy and security, and whether individual trade-offs are justified in the name of public safety. Flock and Ring sell consumers on the idea that their products have the power to not only catch criminals, but also deter them from offending in the first place.

In a 2015 pilot program with the Los Angeles Police Department, Ring said the presence of its cameras reduced burglaries in neighborhoods by as much as 55 percent, a figure that has been disputed.

Is there something about license plate readers you think we should know? Contact the author at louise_matsakis@wired.com or via Signal at 347-966-3806. WIRED protects the confidentiality of its sources, but if you wish to conceal your identity, here are the instructions for using SecureDrop. You can also mail us materials at 520 Third Street, Suite 350, San Francisco, CA 94107.

“It’s in the interest of the jurisdiction and the manufacturer to advance the notion that this is the latest and most sophisticated technology,” says Elias Silverman, professor emeritus at John Jay School of Criminal Justice and the Graduate Center, City University of New York. “One needs to take these advances with a grain of salt and acknowledge they need to be studied by others."

Flock’s cameras are now used in over 400 communities in 35 states, and have been credited with helping police solve a series of serious cases. But the overall effect installing license plate readers has on crime rates still isn’t clear, and likely can’t be determined by conducting a short experiment. Some studies indicate the readers don’t deter crime, while others have found the devices can potentially reduce certain types of offenses.

Maria Cuellar, a professor of criminology at the University of Pennsylvania who researches the use of statistics in the law, says pilots like the one in Cobb County only provide before and after comparisons, which alone can’t prove a causal relationship. “The problem with these is that so many things could have changed between the ‘before’ period and the ‘after’ period,” she adds. That includes everything from the number of cars passing through the area to broader demographic changes. The study was also relatively short. “With such a small sample size in terms of time, any changes could likely be noise rather than an actual signal,” says Cuellar.

Wider trends too have to be taken into account. Police say crime is down overall in Cobb County, as well as in nearby Atlanta. “We do believe that there are other things we are doing that have attributed to the general decline in crime,” says VanHoozer. He notes there are also social factors that might be contributing to the drop, like low unemployment.

Photograph: Flock Safety

Another possibility is that Flock’s license plate readers helped police apprehend a relatively small group of criminals who were responsible for the bulk of the crime in the area. In other words, rather than deterring people from committing an offense in the first place, the license plate readers may have helped apprehend repeat offenders.

Cops have used license plate readers for at least a decade, but the ones made by Flock Safety are arguably more powerful. They automatically catalog a vehicle’s model, color, make, and any distinguishing marks, as well as the date and time they passed through the neighborhood. The cameras ping law enforcement the minute a known stolen vehicle crosses their path, a feature VanHoozer says has been particularly useful in Cobb County. The Flock LPRs are even capable of detecting people walking by, and whether they have a dog in tow.

Keep Reading The latest on artificial intelligence , from machine learning to computer vision and more

There are safeguards in place for how Cobb County police can use the cameras. Flock Safety only allows footage to be retained for 30 days, and according to department policy, officers need to document a legitimate law-enforcement reason and case number to access the tapes. But Flock Safety’s license plate readers are also used by private individuals, who aren’t subject to the same oversight. It’s possible a homeowners' association may use the readers to improperly spy on or surveil its neighbors, for instance.