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.

The 3 are among at least 14 recent murders in Newark in which witnesses have clearly identified the killers but no charges have been filed, infuriating local police commanders and victims’ relatives.

In 8 of the 14 cases, according to court documents and police reports, there was more than one witness; in two of them, off-duty police officers were among those identifying the suspects. But in a DNA era, these are cases with little or no physical evidence, and they often involve witnesses whose credibility could be compromised by criminal history or drug problems, or both.

“No one wants to solve these cases and lock up the killers in these cases more than we do,” the county prosecutor, Paula T. Dow, said in a recent interview. “But we have to weigh the evidence and move forward only if we believe that the witnesses are credible and that they’ll be there to testify at trial.”

The tension between the police and prosecutors here over the evolving standards of evidence required to authorize arrest warrants is a stark example of the profound effect witness intimidation is having on the criminal justice system in New Jersey and across the country.

Surveys conducted by the National Youth Gang Center, which is financed by the federal Department of Justice, have found that 88 percent of urban prosecutors describe witness intimidation as a serious problem.

In both Baltimore and Boston, where “stop snitching” campaigns by rap artists and gang leaders have urged city residents not to cooperate with the authorities, prosecutors estimate that witnesses face some sort of intimidation in 80 percent of all homicide cases.

In Essex County, prosecutors report that witnesses in two-thirds of their homicides receive overt threats not to testify, with defendants and their supporters sometimes canvassing witnesses’ neighborhoods wearing T-shirts printed with the witnesses’ photographs or distributing copies of their statements to the police.

Dozens of New Jersey murder cases have been undone over the past five years after witnesses were killed, disappeared before trial or changed their stories.

In 2004, the Newark police determined that four people found dead in a vacant lot had been killed to silence a witness to a murder; a witness to that quadruple homicide was later shot to death as well.

Ms. Dow, who was appointed in 2003 amid criticism of county prosecutors’ ability to close cases, said she was simply adapting to the evolving code on the streets, where gang violence and widespread distrust of law enforcement have deprived prosecutors of one of the legal system’s most crucial components: dependable witnesses.

The state’s attorney in Baltimore, where witness intimidation is a notorious problem, has taken an even more rigid stand, refusing to file charges in any single-witness case without extensive forensic corroboration.

But that approach differs sharply from those of prosecutors in many other urban areas, like Brooklyn, where the district attorney, Charles J. Hynes, has in recent years taken to reviewing all single-witness cases personally.

In Newark, where the homicide rate has risen in the past few years, the police, local politicians and victims’ relatives are all questioning why prosecutors are holding detectives to a higher standard than the law requires — and letting dangerous criminals remain on the streets.

“How can they leave him out there?” asked Yusef’s mother, Tosha Braswell, referring to the man who shot him. “Are they waiting for him to kill someone else’s son?”

Tension Between Officials

Newark’s mayor, Cory A. Booker, who was elected last year on a promise to reduce crime in the city, recently met with Ms. Dow to ask her to be more aggressive in filing charges. In recent years Newark police officials have accused the prosecutor of being reluctant to take on cases that could be difficult to win because her office was criticized after losing a succession of high-profile trials.

The police director, Garry F. McCarthy, worries that the prosecutor’s approach undermines his crime-fighting strategy of focusing on the small group of criminals responsible for a disproportionate amount of crime.

“The law states the standard for arrest is probable cause, which is different than what is required for conviction beyond a reasonable doubt,” Mr. McCarthy said. “Our goal is to arrest quickly to avoid the potential for additional crime. An arrest does not prevent an ongoing investigation from proceeding.”

Ms. Dow declined to discuss details of any open cases. But she said that she was proud of her office’s performance, and that she hoped her rigorous standards for filing charges would lead investigators to work harder at getting corroboration.

“It’s easy for the police to point fingers when they haven’t done enough detective work to get a conviction,” she said.

In Essex County, the conviction rate for homicides, which includes plea agreements, was 79 percent in 2006, up from 74 percent when Ms. Dow took over three years earlier (but down from 86 percent in 2005 and 83 percent in 2004).

In Baltimore, prosecutors under Patricia C. Jessamy, the state’s attorney, obtained convictions in 65 percent of homicide cases last year, up from 59 percent in 2005 and 52 percent in 2004.

While prosecutors are often measured by such conviction rates, it is difficult to tell through statistics whether they are shying from hard-to-win cases.

What most irks the police is the failure to even file charges in cases in which witnesses have solidly identified a suspect, like the 14 here in Newark over the past three years in which Ms. Dow has declined to authorize arrest warrants. Six of these cases rely on a single witness, including the slayings of Yusef Johnson and Valterez Coley.

Yusef was a football star with a 3.7 grade-point average before he was killed in August 2005. According to police reports, a woman told detectives she had seen the shooting from 30 feet away and was well acquainted with the gunman, a member of the Crips street gang, because he frequently sold cocaine to her.

The case helps illustrate why prosecutors may shy away from single-witness cases: Given the suspect’s status as both a gang member and the witness’s drug supplier, even detectives have their doubts about whether the woman would ultimately testify at trial — or be believed.

On the night Mr. Coley was gunned down near a porch in Newark’s Central Ward, two men told the police they had seen the gunman, whom they identified as a leader of the 252 Bloods street gang. The witnesses said the gunman was looking to settle a score with a young man who had a relationship with his girlfriend, and mistook Mr. Coley for his rival.

But one of the men soon fled the state, leaving the police with a lone witness — and thus no charges have been brought.

Danger in Cooperation

Gregory DeMattia, chief of the Essex County prosecutor’s homicide division, said his investigators saw fallout from witness intimidation every day. When they arrive at a crime scene, he said, bystanders scatter so neighbors will not think they are cooperating with the police.

Those who do help often do so surreptitiously — leaving detectives a note in a trash can or asking to be taken away in handcuffs “so that neighbors will think they’re in trouble with the police and not cooperating,” Mr. DeMattia said.

Prosecutors in other cities tell similar stories about their witnesses being pressured, and say they are cautious about pursuing cases based on lone witnesses because of worries about faulty memory, ulterior motives and, as in the Yusef Johnson case, credibility.

That is part of why Ms. Jessamy, in Baltimore, has all but refused to file charges in single-witness situations.

But across Maryland in Prince George’s County, where there is also a serious gang problem, State’s Attorney Glenn F. Ivey has taken the opposite tack. He insists on pursuing single-witness cases even though he was criticized publicly for losing 4 of them in 13 months.

“If you have a single witness and you believe their story, I believe you’ve got to go forward, even if it’s a case you might lose,” said Mr. Ivey, whose office’s conviction rate on homicides is more than 80 percent. “I’m not going to give the gang members, the murderers and the rapists an easy out. And if they know that all they have to do is get your case down to one witness, I think it would encourage them to use even more intimidation.” Here in Newark, even in cases with multiple witnesses — and occasionally even when one of those witnesses is a police officer — the prosecutor has sometimes been unwilling to authorize arrests.

Take the case of Lloyd Shears, an Army veteran killed during a robbery in December. A man told the police he had seen his neighbor fire the fatal shots. A woman who had been standing next to him told detectives she heard the shots, and then turned to see the neighbor running from the scene. But the neighbor has not been charged.

Or consider the killing of Shamid Wallace, an 18-year-old found face down in the street last August with several gunshot wounds in his torso. Detectives found two witnesses who identified the man they said they saw kill Mr. Wallace. An off-duty Newark police officer heard the gunshots, saw a man fleeing with a gun and later picked the suspect out of a photo array. The suspect has not been arrested.

Then there is Farad Muhammad, who was stabbed to death last July. One witness told the police she had seen someone she knew running away from Mr. Muhammad’s body. An off-duty police officer from neighboring East Orange identified the same man, saying he had seen the suspect chasing Mr. Muhammad with a knife. Again, no charges were filed.

Yusef’s parents, who keep a shrine of photographs surrounding his school sports trophies, still hope that his suspected killer will be arrested soon.

“It’s like they don’t care enough,” said his father, Scottie Edwards, a delivery truck driver.

Wielding Fear Like a Weapon

But to those who suggest Ms. Dow is overreacting to the problem of witness intimidation, her supporters point to the death of Steven Edwards, who was shot and killed in a car on South Eighth Street in January 2006. Within a month, detectives had three witnesses identifying the gunman.

One of the witnesses, a gang member, quickly announced he would never testify for fear he would be ostracized for helping the police — or wind up murdered himself. Six months later, another witness was himself charged in two homicides, shattering any credibility.

The third witness picked the suspect out of a photo array, but immediately began to waver when asked about testifying in open court.

“She would not say she was 100 percent sure,” said a police report on the case, “because she was afraid of retaliation.”