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NEW YORK — New York City is one of the most difficult places in the world to legally obtain a gun, so in the wake of a series of shootings, police are working to crack down on illegal guns.

Over the weekend, a toddler was killed and a 12-year-old boy was shot in separate incidents. Mayor Bill de Blasio on Monday said the gun violence cannot be allowed to continue.

“It begins with the fact that there are just so many guns out there, and that is a New York tragedy and a national tragedy,” he said. “It’s also another indication of the work we have to do. We have to heal. We have to bring our police and our communities together.”

In 1993, the NYPD estimated about 2 million guns circulated around the city. Police believe about the same number of guns still come through the iron pipe line from the South where gun laws are lax.

Dr. Rob Gonzalez, a criminal justice professor at St. John’s University, pointed to the June dissolution of the city’s anti-crime unit.

“They’re still actually focusing on where the guns are coming from, particularly from the South, however we have to focus on street level violence and it’s the anti-crime unit or lack of anti-crime unit that’s causing this uptick,” he said.

In 15 of the most violent precincts in the Bronx, Brooklyn and Queens this year, there have been 870 gun arrests and 492 of those arrests were done by anti-crime units. So the units were responsible for about 50 percent of gun arrests.

Retired NYPD Deputy Inspector Corey Pegues agreed that the anti-crime unit played a large role in getting guns off the street.

“Anti-crime is the preeminent gun catchers on the streets in the city of New York,” Pegues said. “The community can’t have cake and eat it, there was an uproar with stop, question and frisk and now they dismantled the unit. Crime is on the uptick and everybody’s screaming to have them back.”

The anti-crime unit had more weakness than strengths and caused more trouble in communities of color thangood, critics said.

“He [NYPD Commissioner Dermot Shea] came to the decision that the anti-crime unit, that work could be done a different, better way,” de Blasio said. “A way that uses modern technology, a way that uses the skills of officers to achieve the same outcomes to get those guns off the street, but in ways that also don’t disrupt the relationship between police and community.

Starting Monday, 600 members of anti-crime are moving into new roles. Many are going to detective squads. Pegues still has questions about the change.

“Why did you take them from those violent precincts? If you wanted to do it, do it smartly,” Pegues said.