Not long ago, New Yorkers were required to wear a face mask if they wanted to enter a store. But Mayor Eric Adams has now said the city’s shopkeepers should adopt the opposite approach: People who refuse to pull down their mask when they first come into a store should be barred, in case they plan to rob the place.

“We are putting out a clear call to all of our shops, do not allow people to enter the store without taking off their face mask,” the mayor said in a radio interview on 1010 WINS on Monday. “And then once they’re inside, they can continue to wear it if they so desire to do so.”

The mayor made the suggestion, which an aide to Mr. Adams said is not a legal requirement or even a new idea, at a time when crime appears to be declining. The city has seen reduced rates of crime across most categories, from murder to petit larceny, for the first two months of 2023 compared with the same period last year, Police Department data shows.

But theft has been a harder problem to address, and concerns over robberies and shoplifting persist. The problem was underscored over the weekend by the shooting death of a 67-year-old deli employee in the Bronx, who was killed on Friday night by a robber wearing a dark face mask and a white, full-body Tyvek suit.

The mayor referred to that killing in a second news media appearance on Monday.

“Let’s be clear, some of these characters going into stores that are wearing their mask, they’re not doing it because they’re afraid of the pandemic, they’re doing it because they’re afraid of the police,” the mayor said in a television interview with PIX 11. “We need to stop allowing them to exploit the safety of the pandemic by wearing masks, committing crimes.”

Mr. Adams’s suggestion is sure to draw the ire of those who remain committed to masking in public as Covid persists, as well as those with health conditions that put them at an elevated risk of severe illness if they were to be infected with the coronavirus.

Mr. Adams won election in 2021 in part by promising to crack down on rising crime, which afflicted the city during the height of the pandemic. But since taking office, he has continued to sound the alarm even as city data has indicated that New York has become safer.

Despite the wide-ranging recent decline in crime, the number of robberies fell by only 2 percent, city data shows. And the number of petit larcenies — the theft of items whose value is low, such as those typically taken in a shoplifting — increased by about 5 percent compared with the same period last year.

Requiring shoppers to briefly drop their face masks would allow security cameras to get a clear view of a shopper’s face, the mayor said. And if a shopper refused to comply, Mr. Adams said store employees would know to keep an eye on them because “if someone is violating the basic rules, they may be there to violate a substantial rule as to commit a crime.”

“That’s all we are trying to do — empower the store owners, the storekeepers, so they can play a role that we are going to do as police officers,” he said.

Jeffrey Maddrey, the Police Department’s chief of department, echoed the call for shoppers to drop their masks during a news conference outside Daona, the deli in the Bronx where the employee was killed on Friday. His name has not been released.

Chief Maddrey said New Yorkers should think of dropping their masks as “a peace offering” and “a sign of safety for store workers.”

“When we walk in, we should take down our masks,” the chief said. “We should let them know that they’re not in any danger, any harm, that we’re customers, we’re here to help them.”

But as a crime prevention strategy, he acknowledged it might have shortcomings.

A store that keeps its door locked and admits shoppers via a buzzer is better able to enforce a mask-off rule, he said, while a store that keeps its doors open has less control.

Nevertheless, the idea has been embraced by the United Bodegas of America Association, an umbrella group for the corner stores that are ubiquitous across the city.

Fernando Mateo, a representative of the association, attended the news conference at Daona wearing a hospital mask, a beanie and a hood so that only his eyes were visible.

“This is what we’re talking about,” he said. “This is what we don’t want.”

“We’re talking about all those people who hoodie up, that cover their entire face so that the Police Department cannot identify them when they commit a crime,” he added. "These criminals have gotten so comfortable that they’re doing it repeatedly, and things get out of hand and people are killed.”

Hurubie Meko, Brittany Kriegstein and Emma G. Fitzsimmons contributed reporting.