“People think this is something new,” the county sheriff, Bob Gualtieri, said of facial recognition. “But what everybody is getting into now, we did it a long time ago.”

A Question of Due Process

Only one American court is known to have ruled on the use of facial recognition by law enforcement, and it gave credence to the idea that a defendant’s right to the information was limited.

Willie Allen Lynch was accused in 2015 of selling $50 worth of crack cocaine, after the Pinellas facial recognition system suggested him as a likely match. Mr. Lynch, who claimed he had been misidentified, sought the images of the other possible matches; a Florida appeals court ruled against it. He is serving an eight-year prison sentence.

Any technological findings presented as evidence are subject to analysis through special hearings, but facial recognition results have never been deemed reliable enough to stand up to such questioning. The results still can play a significant role in investigations, though, without the judicial scrutiny applied to more proven forensic technologies.

Laws and courts differ by state on what investigative materials must be shared with the defense. This has led some law enforcement officials to argue that they aren’t required to disclose the use of facial recognition.

In some of the Florida cases The Times reviewed, the technology was not mentioned in initial warrants or affidavits. Instead, detectives noted “investigative means” or an “attempt to identify” in court documents, while logging the matters as facial recognition wins in the Pinellas County records. Defense lawyers said in interviews that the use of facial recognition was sometimes mentioned later in the discovery process, but not always.

Aimee Wyant, a senior assistant public defender in the judicial circuit that includes Pinellas County, said defense lawyers should be provided with all the information turned up in an investigation.