Citywide, violent crimes have dropped to record lows. But in some neighborhoods, crime is stubbornly persistent. My colleague Ali Watkins wrote about shootings in Bedford-Stuyvesant, Crown Heights and other parts of northern Brooklyn, and what the police are doing to stop the violence.

How violent are these pockets of crime?

In northern Brooklyn, there have been 11 more shootings so far this year than there were a year ago. Over that same period, northern Queens had 10 additional shootings and northern Manhattan had 16 more.

What is behind the violence?

One answer is access to guns. Though New York City’s gun laws are restrictive, other cities and states have looser regulations. For years, law enforcement officials have complained about how guns illegally flow into New York with disturbing regularity.

Another answer is gangs.

Not all of the shootings in northern Brooklyn last weekend were gang-related, but at least one was part of what the police said was tit-for-tat retaliatory gang activity. The gangs, according to the police, include the Brisp Nation, Gates Fam, Rich Fam, the Hoolies and the 900 gang.

Not familiar with these gang names?

The gangs are often tied to housing developments and individual blocks, rather than to national or regional groups, the police and community groups say.