Instead, the case stands as a striking example of the way witness intimidation has stymied law enforcement and allowed gangs to tyrannize entire communities. The truce quickly unraveled. The charges against the two gang members were dropped within a month. Even a local program designed to coax young men out of gangs by buying them business suits has seen its limitations; one participant had his outfit designed in Bloods red.

Ten months after Tajahnique was wounded, 18-year-old Naquan Archie was shot and killed on the same corner during a robbery that the police believe was carried out by a member of the Bloods. Neighbors and detectives say there were at least three witnesses, but none have identified the gunman.

“I watched my nephew die in my apartment,” said Mr. Archie’s aunt, so terrified of retaliation that she would speak only on the condition her name not be published. “People saw him get shot. But they know what’s going to happen if they talk. There’s nothing anyone can do about it.”

Such silence has spread over the last decade in cities across the country, as the proliferation of gangs like the Crips, Bloods and Latin Kings has made witnesses an endangered and elusive component of countless criminal investigations. Criminologists say gang culture has made fair game of brutally punishing anyone who helps the police. What results is a self-perpetuating cycle of intimidation and helplessness: residents refuse to risk their lives by helping a police force that cannot protect them; the authorities say they are powerless to lock up gang members without witnesses willing to testify.

In Trenton, a city of 85,000 where the police estimate that the Bloods have as many as 2,000 members, overall crime is down and officials say violence is largely confined to areas where gangs are most prevalent. But gang killings remain a persistent problem. There were 20 homicides in the city last year; the police have made arrests in nine of the 16 killings they consider gang related, and in three of the others. In the first half of this year, murders increased by 50 percent.