It is unclear when the face-scanning system was installed. The people familiar with the Garden’s use of the technology, who were granted anonymity because they were not authorized to speak publicly about it, said they did not know how many events at the Garden in recent months have used it or how the data has been handled.

“In a lot of places we will see facial recognition framed positively as just an extension of video surveillance,” said Clare Garvie, an associate at the Center on Privacy and Technology at Georgetown Law Center. “But the reality is it is a way to require, or in secret, have everyone in a crowd show their papers, essentially, to compare them to a big enough database.”

The Garden — home to the N.B.A.’s Knicks and the N.H.L.’s Rangers, and host to events like boxing matches, concerts and the Grammy Awards — was already known for having tight security. There is always a heavy police presence in part because the arena is in the heart of Midtown Manhattan and is built above Pennsylvania Station, the nation’s busiest rail terminal. Fans attending events go through security screening that can include metal detectors, bag searches and explosive-sniffing dogs.

The use of facial recognition technology puts the arena in the vanguard of professional sports facilities. At least two other arenas have experimented with the technology, but teams and leagues are generally unwilling to discuss security protocols, so it is difficult to know for sure how widespread it is.

“Nothing is more important to us than the safety and security of the fans, players, team and arena staff at our games,” said Mike Bass, a spokesman for the N.B.A. “The league and our teams are exploring the use of all state-of-the-art technology, including facial recognition, to ensure that we have industry-best security measures to protect all those in our arenas.”