NEW YORK >> Madison Square Garden has quietly used facial-recognition technology to bolster security and identify those entering the building, according to multiple people familiar with the arena’s security procedures.

The technology uses cameras to capture images of people, and then an algorithm compares the images to a database of photographs to help identify the person and, when used for security purposes, to determine if the person is considered a problem. The technology, which is sometimes used for marketing and promotions, has raised concerns over personal privacy and the security of any data that is stored by the system.

“MSG continues to test and explore the use of new technologies to ensure we’re employing the most effective security procedures to provide a safe and wonderful experience for our guests,” MSG said in a statement.

A spokeswoman for the Garden declined to answer questions about the use of face-scanning technology.

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 NBA’s New York Knicks and the NHL’s New York Rangers, and hosts 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 NBA. “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.”

The NHL declined to comment.

Although security is the most obvious use of the technology, some independent experts say it is less effective as a security measure for private businesses because they do not have access to various watch lists held by law enforcement agencies. In fact, some vendors and team officials said the customer engagement and marketing capabilities of facial recognition are even more valuable than added security for sports facilities.

Law enforcement agencies have used facial recognition technology for many years, but some commercial entities have been wary. Walmart is among those that have experimented with it, to help identify shoplifters, drawing strong objections from privacy groups.

The software can be used to determine who is allowed into a building, like vendors or workers at a specified employee entrance. Companies may eventually be able to use the technology to increase customer engagement. In the case of an arena, a sports fan might sign up for a loyalty program with a team and attach their image and a credit card to the account. They could then park without paying an attendant, walk in without having a ticket scanned and pay for merchandise and concessions without ever taking out their wallet.

Even without fans signing up for anything, the cameras can give teams a much better sense of who is attending a game. Currently, teams might know who originally bought a ticket, but after the ticket enters the secondary market, teams do not necessarily know who is sitting in the seat.

“The days of having 40,000 to 60,000 people in the stadium and not knowing who they are, I think those days are going to disappear,” said Charles Carroll, a senior vice president at IDEMIA, which manages the Transportation Security Administration’s PreCheck program. IDEMIA has partnered with three sports venues on security, including Barclays Center in Brooklyn, to offer expedited lines to enter.

In addition to Madison Square Garden, at least two other arenas are known to be experimenting with the technology. According to a Sacramento Kings spokeswoman, facial recognition is used to allow players and staff to enter the practice facility connected to the Golden 1 Center, but its use has not expanded to event attendees.

The Dallas Mavericks have contracted with Suspect Technologies to experiment with facial recognition outside the team’s locker room and throughout the American Airlines Center.

Mavericks owner Mark Cuban, however, said in an email the team needs to “find the right application that creates so much value people want to use it.” He said that, for now, facial recognition doesn’t improve his arena’s ability to keep out unwanted patrons enough to justify its implementation.

“In the private sector, facial recognition is really only as good as the database it is compared against,” said Michael Downing, former deputy chief of the LAPD and chief security adviser for the Oak View Group.

There is no federal law governing the use of facial recognition technology, though both Illinois and Texas have laws that restrict its use without informed consent. Facebook has been sued under Illinois’ law, a case that could challenge its business model, and according to the Center for Public Integrity, is lobbying against similar laws being passed in other states.

“We are in a kind of legal Wild West when it comes to this stuff,” said Jay Stanley, a policy analyst at the American Civil Liberties Union. While most sports fans may not have a legal right to know their face is being tracked, Stanley said he believed there is an ethical right to know.

“I should know if I am being subject to facial recognition if I am going into any business, including a stadium,” he said. “Even if you are just running my face against a list of people who have been banned from the premises and doing nothing else with it. I want to know. I have a right to know.”

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