Police in the small Maryland city of Hagerstown used a cutting edge, facial recognition program last week to track down a robbery suspect, marking one of the first such instances of the tactic to be made public.

In the process of identifying a possible suspect, investigators said they fed an Instagram photo into the state’s vast facial recognition system, which quickly spit out the driver’s license photo of an individual who was then arrested.

This digital-age crime-solving technique is at the center of a debate between privacy advocates and law-enforcement officials: Should police be able to use facial recognition software to search troves of driver’s license photos, many of which are images of people who have never been convicted of a crime?

An increasing number of police departments across the country are running images through driver’s license databases in their investigations. But the Hagerstown case is one of the few resulting in an arrest that has become public, experts in the field say.

Thirty-one states now allow police to access driver’s license photos in facial-recognition searches in addition to mug shots, according to the Center on Privacy and Technology at the Georgetown University Law Center. Roughly one in every two American adults—117 million people—are in the facial-recognition networks used by law enforcement, according to a 2016 report by the center.