Prince William County prosecutors wrestled with a tricky question last year when they were preparing to persuade jurors to recommend a death sentence for convicted sniper John Allen Muhammad.

Should Muhammad's cellblock mate -- a convicted child rapist and avowed devil worshiper -- testify in court to prove the sniper was an escape risk and should therefore be executed? Russell Smith could tell jurors alarming details about Muhammad's plans to tackle a guard, rip off his uniform and waltz right out of the Prince William jail.

But neither his testimony nor his statement was ever used. Prosecutors felt Smith's background would be rich fodder for Muhammad's defense attorneys, who could easily attack Smith's credibility and then persuade jurors to spare the sniper's life.

"All cooperative witnesses have baggage and have something the defense can point to," said Commonwealth's Attorney Paul B. Ebert. "What Smith had to say wasn't going to help us as much as his background was going to hurt us. I've told juries that you've got to pet a skunk in order to catch another one. We just didn't want to pet this one."

Prosecutors secured a death sentence for Muhammad without Smith's help, but the decision they faced illustrates a dilemma in the game of bringing down big criminals: to use or not to use people charged with crimes as witnesses or undercover agents to win indictments. Even though criminals may have a minimum of credibility in a juror's eyes, law enforcement officials say they must rely on them to nab society's more flagrant scofflaws.

Cooperative witnesses or informants are primarily the lower-level conspirators who get arrested but agree to testify against their ringleaders or tape-record conversations to get charges dismissed or a lighter sentence.

Prosecutors say tapping the services of criminals can be very effective -- or can backfire.

In August, a Prince William man who helped authorities in their pursuit of a large OxyContin drug conspiracy in exchange for a reduced sentence was charged in the death of his housemate's baby. Stephen Wendel, 35, had tape-recorded conversations with a major target that could help form the basis of a federal indictment. He had also done an undercover buy of OxyContin that could help build a criminal complaint in Prince William.

Wendel's "substantial assistance," as his stealthy deeds are formally known in the courtroom, saved him as much as 20 years in prison and a $1 million fine, the maximum federal penalty for conspiracy to distribute OxyContin. In January, he was sentenced to three years of supervised probation.

But the fact that he has now been charged with murder in Prince William County may violate his federal probation and could diminish the impact of his "substantial assistance."

"It could come into question, especially if the government intends to call him as a witness," said James A. Willett, assistant commonwealth's attorney for Prince William County. "I doubt that his contribution was so central to the case, but certainly if the federal government chose to use him as a witness, that would bear on his credibility."

Wendel's federal public defender, Suzanne Little, and Assistant U.S. Attorney Gene Rossi declined to comment on his case.

Orin S. Kerr, a criminal law professor at George Washington University, said cooperative witnesses can be a "very powerful tool but a dangerous one" because "any time the law creates an incentive for a criminal to appear very helpful to the police, you need to question the credibility of the evidence."

Montgomery County State's Attorney Douglas F. Gansler said cooperative witnesses are used about 25 to 30 percent of the time in conspiracy cases. In more suburban or rural areas, investigators will rely more on police to do undercover work because jurors there typically trust law enforcement officers more than they do in such urban places as the District, he said.

"There are literally people in Washington, D.C., who think police drive around with bags of cocaine, planting it on people," said Gansler, who was an assistant U.S. attorney in the District in the 1990s.

Cooperative witnesses or informants are typically a factor in drug conspiracy cases, when police use drug addicts to get to the more powerful dealers and suppliers. Wendel, for instance, was a cog in a large network of up to 80 doctors and patients who were illegally using and selling the powerful OxyContin painkiller in Virginia and elsewhere, according to court documents.

When such people as Wendel are arrested, they instantly become valuable, and if they decide to go undercover, they must be watched vigilantly, said Aaron Graham, vice president of security for Purdue Pharma, the manufacturer of OxyContin.

Graham said a low-level player in a conspiracy initially will refuse to help police, fearing retribution or believing he doesn't need to cooperate because his co-conspirators still on the loose will watch over his relatives and personal affairs.

"Then they'll call back from jail and say, 'My guys haven't taken care of my family, so I am going to help,' " said Graham, who spent several years working undercover in Mexico and California as a special agent with the Drug Enforcement Administration. "But any time you use a cooperative individual on an undercover job, there's a whole new set of rules. You have to search him completely" and make sure he doesn't do any unsanctioned deals.

Prince William business owner Rick Reeder said that when his son Brian was arrested this year for allegedly being part of a cocaine ring, he was resentful of all the confidential informants who ratted his son out.

"But my biggest complaint is that police had been investigating it for so long and Brian didn't get into it until late," Reeder said. "They let it keep going to get more people."