In 2007, Lanier said, there were 70 gangs involved in criminal activity. Now there are about 20. In 2008, there were 142 homicides involving guns. Last year, there were about 56 — a sign, police say, that there are fewer guns on the streets.

And several years ago the city paid out $200,000 for tips leading to arrests and convictions in homicide cases. Last year, it awarded $575,000.

“People walking around saying ‘we can’t prevent homicides’ is one of the most frustrating things,” Lanier said. “We can prevent homicides.”

Gang unit officers targeted certain areas and let gang members know that they were being watched. If a gang fight broke out in a school or on Metro, officers would be on members’ doorsteps hours later, telling them they would go to jail if they retaliated.

The department has emphasized technology. The District has the country’s biggest deployment of ShotSpotter gun sensor technology, which alerts police to the sound of gunfire. Lanier also oversees the department’s license plate readers, scores of cameras fanned across the city that spot wanted cars and monitor vehicles traveling in and out the District.

“You can’t arrest your way out of crime problems,” said Prince George’s Police Chief Mark Magaw. “You’ve got to get at the core of what’s driving those issues.”

Peter Hermann, Matt Zapotosky, Clarence Williams, Justin Jouvenal, Michael Laris, Caitlin Gibson and Jeremy Borden contributed to this report.