When a friend was gunned down on the North Side in September, 19-year-old Justen Anderson did what few do in his situation -- he stepped forward.

Mr. Anderson, who was standing next to Jason Brown, 30, when he was shot, broke the culture of silence to give police information that led to an arrest warrant for the alleged killer, Devon Shealey, 22.

On Wednesday, hours before police finally caught Mr. Shealey, Mr. Anderson was killed by a bullet that sailed through a second-floor window of his grandmother's Perry South home.

Police said it's too early to tell whether Mr. Anderson was shot because he was a witness, but the possibility gives new fuel to bystanders' paralyzing fear of aiding authorities when violent criminals strike.

Mr. Anderson is the second Pittsburgh homicide witness killed in recent months.

This week, Allegheny County Common Pleas Judge Jeffrey A. Manning will decide whether to allow jurors in the homicide trial of Donald "Tank" Wilson to hear the preliminary hearing testimony of Korey Johnson. Mr. Johnson fingered Mr. Wilson in the April killing of 12-year-old Kholen Germany in Wilkinsburg, then was shot and killed himself in September outside a Point Breeze gasoline station.

A woman in the car with Mr. Johnson when he was shot told police the shooter was 19-year-old Jesse Engram. But at a preliminary hearing she recanted her testimony, saying she couldn't make out the killer.

Mr. Engram, nonetheless, was held for trial, but the shifting story could compromise prosecutors' case before a jury.

A hesitant witness like this one is a frequent phenomenon.

"They feel that there will be retaliation, and in some instances there have been," said Sarah Campbell, of Homewood, the president of the Zone 5 Public Safety Committee, which meets with police to discuss issues in the community.

"There are people who are invisible, who have power that we will never see, and we will never understand why someone disappeared. It's not a simple matter."

The witness in the Engram case told police that Mr. Johnson had been branded with the dangerous epithet of "snitch."

"It's difficult to investigate a case because of that," said Pittsburgh police Major Crimes Lt. Dan Herrmann.

"People do not want to cooperate because they're going to be labeled a snitch. They're afraid they will be harmed in some way."

For those who might be in danger, the city offers a witness protection program, which has a budget this year of $87,000, funded in part by the Allegheny County district attorney's office. County police also have a program to protect witnesses.

Detectives are given credit cards to use on any witness who might be in danger, regardless of the crime. Witnesses can be put up in hotels and have their meals paid for as detectives assess risk and devise long-term plans.

District Attorney Stephen A. Zappala Jr. said detectives work with witnesses, in some cases, to find them new jobs and take care of other needs like children or substance abuse problems.

"We have a program that can keep you safe," Mr. Zappala said.

"It's a question of trust. 'Do I believe that you care about me and will protect me beyond the time of trial?' Say you get a conviction and all the defendant's associates are identified. Then you do an organizational risk assessment to see if other things should be done."

Investigators also take pains to keep witnesses' identities a secret, though word frequently has a way of leaking out.

"You can stand up there and tell people, 'We'll protect you if you tell us what you know,' " Ms. Campbell said. "And a person knows that if they tell, that somebody is listening, and they can't get them out of town quick enough. It's a two-sided sword."

Still, authorities say murders of witnesses are rare, happening only a few times per decade.

No one under protection from the city's program, which started in the early 1990s, has been killed. And if a witness does get killed, it typically is unrelated to their testimony.

Both Mr. Johnson and Mr. Anderson turned down protection, like many witnesses who don't want to be uprooted.

Mr. Anderson's uncle, Leon Moore III, said Mr. Anderson didn't want to leave his grandmother, Robeit Baldwin.

"Protection programs are fine, but only when you're looking at them on television, because it's the person's total life that you've got to look at," Ms. Campbell said.

"I'm going to lose all my friends. I lose contact with my family. I'm going to be somewhere. Who's going to be my friend?"

Mr. Zappala acknowledged the fear and lack of trust for potential witnesses. Although the office is constantly working to improve the protection program, he said there is an increasing effort to move beyond witnesses as a means of making cases.

Aside from being difficult to come by, witnesses often don't stand up as credible to jurors -- especially when their stories shift throughout an investigation.

Mr. Zappala said DNA evidence and surveillance footage will become more prevalent as the technology improves, reducing the need for witnesses to testify at trial.

The usefulness of such tools was evident last month when a jury convicted Terrence Allen, 27, of Knoxville, in the 2006 murder of his stepfather.

The prosecution's key witness, the defendant's mother, Trudy Williams, backed down from her initial statements to police and testified that she couldn't recognize the killer. But surveillance video from a nearby school captured the shooting, and jurors could watch an unafraid Ms. Williams plead with the shooter to stop -- something she likely would not have done had he been an unknown intruder.

"There are other ways to make a case," Mr. Zappala said.

Yet witnesses remain crucial to authorities' efforts to prosecute criminals. And there aren't a whole lot of Justen Andersons around.

"He was trying to do the right thing," said Pittsburgh police Cmdr. Thomas Stangrecki.

"As in all of these cases, sometimes when no one comes forward, we're left with a killer on the street."