The New York Police Department officer who was killed in a shooting in a Midtown Manhattan skyscraper on Monday was working as part of the Paid Detail Unit, which was created in 1998 to allow uniformed officers to work as security guards while off duty.

Under the program, companies can hire officers for shifts in office building lobbies, stores, sports complexes, banks and other venues that are open to the public, keeping them safer and increasing police visibility.

It is often used by large corporations, said Kathryn S. Wylde, leader of the Partnership for New York City, a business group.

“It’s considered an expensive, but very useful, upgrade of security,” Ms. Wylde said, adding that its use grew after the Sept. 11 attacks and, later, during the Covid-19 pandemic. “Small businesses want to use it and can’t afford it.”

When the system started, inspired by similar programs in other cities, it let New York officers work in uniform off the clock for the first time, former officers said. For many, it was their first time holding a second job.

“You put your name on a list, and when a job comes up, they call you,” said John McAuliffe, who retired from the force in 2020 and once worked security at a Yankees game under the program.

The Police Department vets the businesses, which provide direct payments to the officers. The department takes out a 10 percent administrative fee. Officers working the shifts are not able to issue summonses, but are expected to follow department rules, according to the Police Department.

The department did not immediately respond to a request to comment on the program. As of 2016, officers could make $41 an hour, sergeants $51 and lieutenants $57.

“Up until yesterday, it was like easy money,” Mr. McAuliffe said.

Randolph Blenman, who retired from the force in 2004, recalled working shifts at Toys “R” Us and at the 2000 World Series between the Yankees and Mets. Many officers embraced the opportunity to make good money on lower-stress shifts, he said.

“You weren’t standing on the corner of 125th Street and Lex,” he said, referring to a busy and sometimes chaotic corner in East Harlem. “They would have people at Rockefeller Center during the holidays.”

Richard M. Aborn, the president of the nonprofit Citizens Crime Commission, said the system has offered benefits to the city and to officers.

“It’s a smart way of increasing police presence in the city,” he said. “And it gives the cops a chance to earn extra money.”

Maria Cramer contributed reporting.