NEWARK, Feb. 27  When Yusef Johnson, a 15-year-old honors student, was killed outside an apartment complex here so gang-infested it is known as Crazyville, a witness came forward within days and told the police she knew the man she had seen fire the fatal shots.

In another case three months later, in November 2005, officers found two people who identified a street gang leader as the man they saw kill a marijuana dealer named Valterez Coley during a dispute over a woman.

And when Isaiah Stewart, a 17-year-old wearing an electronic monitoring bracelet from a recent brush with the law, was gunned down that December, another Newark teenager sketched a diagram of the crime scene, correctly identified the murder weapon and named a former classmate as the person he had watched commit the crime.

They seem like slam-dunk cases, but none of the three suspects have been arrested. It is not that detectives are unsure of their identity or cannot find them. Rather, it is because so many recent cases here have been scuttled when witnesses were scared silent that the Essex County prosecutor has established an unwritten rule discouraging pursuit of cases that rely on a single witness, and those in which witness statements are not extensively corroborated by forensic evidence.