“Although we understand that it’s not a 100 percent accurate technology yet, it’s still evolving,” said Tony Montoya, the president of the association. “I think it has been successful in at least providing leads to criminal investigators.”

Mr. Cagle and other experts said that it was difficult to know exactly how widespread the technology was in the United States. “Basically, governments and companies have been very secretive about where it’s being used, so the public is largely in the dark about the state of play,” he said.

But Dave Maass, the senior investigative researcher at the Electronic Frontier Foundation, offered a partial list of police departments that he said used the technology, including Las Vegas, Orlando, San Jose, San Diego, New York City, Boston, Detroit and Durham, N.C.

Other users, Mr. Maass said, include the Colorado Department of Public Safety, the Pinellas County Sheriff’s Office in Florida, the California Department of Justice and the Virginia State Police.

U.S. Customs and Border Protection is now using facial recognition in many airports and ports of sea entry. At airports, international travelers stand before cameras, then have their pictures matched against photos provided in their passport applications. The agency says the process complies with privacy laws, but it has still come in for criticism from the Electronic Privacy Information Center, which argues that the government, though promising travelers that they may opt out, has made it increasingly difficult to do so.

But there is a broader concern. “When you have the ability to track people in physical space, in effect everybody becomes subject to the surveillance of the government,” said Marc Rotenberg, the group’s executive director.

In the last few years, facial recognition technology has improved and spread at lightning speed, powered by the rise of cloud computing, machine learning and extremely precise digital cameras. That has meant once-unimaginable new features for users of smartphones, who may now use facial recognition to unlock their devices, and to tag and sort photos.