Research conducted in surveillance-heavy European cities shows that such cameras don’t deter crime, she said. “It often displaces crime, so it can move it to another area of the city, and can help in solving crimes.” Superintendent Harrison nodded. During the year that the cameras have been active, he’s already seen violent-crime hot spots shift. “I would agree that it doesn’t always deter crime, but sometimes displaces it,” he said.

Superintendent Harrison said that because the New Orleans Police Department was under a federal consent decree, his staff had been working with the decree’s monitor to create and carry out policies that ensured that camera images could be uploaded only to a secure police-evidence cloud, not downloaded by an operator. The decree very specifically limits monitoring to local officials and precludes sharing surveillance with immigration officials, he said; federal law enforcement partners have to request access for specific crimes that have been committed.

To start, the images can be viewed only from a computer screen in the monitoring center when police dispatchers receive a 911 call, he said. “And then we’re monitoring the people who are monitoring to make that sure they’re not abusing that.”

“What you’re describing, of how it turns on once there’s an incident, is actually something we know already in policing — it’s called ‘the probable-cause standard, right?’” Ms. Angwin said. “But what is the civilian oversight or the judicial oversight to make sure that’s really true?”

Also, Ms. Angwin asked, did everyone in New Orleans know how to deal with images generated by the camera system? “We have a clear system in the offline world,” she said. Probable cause is clearly defined in court, for instance. “And we know what happens afterward: how to challenge that warrant in court.”

Dealing with legal matters generated from online images is much more murky, she said. “This is a situation where it’s totally outside of those rules, right? And so I think what we face as a society is building that set of rules.”