"It's a fine line where you need to protect the rights of the citizens, but you also are protecting the right of citizens when you ferret out crime," said Anthony Silva, administrator of Rhode Island's Division of Motor Vehicles and a former town police chief.
Establishing identity, Silva said, is essential to effective police work: "I can't tell you how many times I was handed fraudulent documents. And when you are on the street at 3 a.m., who do you call?"
Pennsylvania's Justice Network, which has allowed police anywhere in the state to compare a facial image with mug-shot databases, has become a key investigative tool, officials said, and last month it added access to 34 million driver's-license photos. (Some residents have several images, taken over years.)
A detective in Carlisle, Pa., attempting to learn the real name of a suspect known on the street as "Buddha the Shoota" compared a Facebook page picturing the man with the mug-shot database and got a promising lead.
"Facebook is a great source for us," said Detective Daniel Freedman, who can do facial searches from his department-issued smartphone. "He was surprised when we walked in and said, 'How you doin', Buddha?' "
He said the suspect responded, "How you know that?" — to which Freedman replied simply, "We're the police."
There typically is little concern when facial-recognition systems relying on criminal databases help identify suspects in narrowly targeted investigations. But searches against images of citizens from driver's licenses or passports, as opposed to mug shots of prisoners, raise more complex legal questions.
Police typically need only to assert a law enforcement purpose for facial searches, whether they be of suspects or potential witnesses to crimes. Civil libertarians worry that this can lead to broadly defined identity sweeps. Already many common but technically illegal activities — blocking a sidewalk, cycling at night without a light or walking a dog without a leash — can trigger police stops and requests for identification, they say.