Nearly 60 percent of white New Yorkers approve of the tactic. Less than a quarter of black New Yorkers share that opinion. Many perceive it as racial profiling, an interpretation bolstered by recent police testimony in a lawsuit filed against the city over the practice. This perception has real-world consequences, says David M. Kennedy, a criminologist at the John Jay College of Criminal Justice. In high-crime neighborhoods across the country, community cooperation with police investigations has virtually stopped. It’s not simply that residents are afraid of retaliation, Kennedy says. “There is a strong and growing norm in many communities, especially poor black communities, that good people don’t talk to and don’t work with the police,” he says. So even while the sheer number of murders in most cities is dropping, the homicide clearance rate — the proportion of cases solved — is doing the same.

Cities grappling with gang violence have long feared one outcome above all others: becoming Los Angeles, and more specifically, Watts. Nowhere were police relations with the black community worse.

The way the L.A.P.D. conducted itself in South Los Angeles “wasn’t policing, it was anti-insurgency run amok,” says the journalist and historian Joe Domanick. “Sheer brutality, suppression and force — those were the only things the L.A.P.D. thought people in South L.A. understood, and those were the only things the L.A.P.D. itself understood.” The rise of the Crips and the Bloods in the 1970s only strengthened that sentiment. Watts, with the highest concentration of public housing west of the Mississippi, the fourth-highest concentration of poverty in the city of Los Angeles and a long history of police-community conflict, presented the problem of gang violence and “black-blue” antagonism in its most extreme form.

But Watts and the Los Angeles Police Department have each undergone a remarkable transformation. Over the past two years, violent crime in Watts’s public-housing projects has fallen by more than 60 percent. Drive-by shootings, once a mainstay of gang life and the nightly news, have almost completely disappeared.

Causality is slippery, especially when it comes to crime. The L.A.P.D.’s decision to deploy 30 additional officers to Watts’s three largest housing projects has undoubtedly contributed to the area’s improvement. Research has shown that “hot-spot policing” — flooding high-crime areas with police officers — effectively reduces crime without simply displacing it. But the department’s efforts in Watts go beyond “cops on dots.” In recent years, the L.A.P.D. has been conducting an unusual experiment in community policing in Watts. Its centerpiece, the Community Safety Partnership, is the department’s collaboration with a group of residents known as the Watts Gang Task Force. Every Monday morning, community leaders meet with top police commanders to discuss what’s happening in the Watts gang world — who’s feuding with whom, where criminal investigations stand, which are the issues residents are worried about. What makes the initiative unusual is that many of the task force’s participants have close ties to street gangs. Some, like Mendenhall, are former gang leaders. Others are the mothers and grandmothers of notorious gang leaders past and present.

When we talk about crime, we tend to talk about victims and offenders, innocence and guilt, prey and predator. Gang violence clouds and warps this logic: victims and victimizers are often the same people, and neither side has any reason to talk to the police. This presents a conundrum to law enforcement, one that has developed contrasting strategies on either coast. New York City insists that hard-nosed, divisive tactics like its stop-and-frisk policy are necessary to reduce crime. But Los Angeles has pursued another way, an approach that has delivered lower crime rates and fostered police-community reconciliation.

Cynthia Mendenhall was born in Shreveport, La., in 1962. When she was 3, her mother, Bobbie Sue Mendenhall, moved to Watts, joining the great migration of African-Americans from the South to Southern California, drawn by the promise of factory jobs and lives free of Jim Crow. Mendenhall’s family grew to include seven children, but it did not thrive. By the time the Mendenhalls arrived, the same year as the Watts riots, the factories along Alameda Street that nurtured a generation of black Angelenos were already beginning to close down.