For decades, in cities from coast to coast, FBI agents recruited killers and crime bosses as informants, and then looked the other way as they continued to commit violent crimes.

When the practice first came to light in Boston -- unleashing an ongoing investigation that has sent one agent to prison for obstruction of justice -- FBI officials in Washington portrayed it as an aberration.

But interviews with nine former FBI agents -- men with a combined 190 years of experience in more than 25 bureau offices from Texas to Chicago and from Los Angeles to Washington -- indicate the practice was widespread during their years of service between the late 1950s and the 1990s.

The former agents, and two federal law enforcement officials who have worked closely with the bureau, said the practice sometimes emboldened informants, leading them to believe they could get away with almost anything.

The degree to which the practice continues today is unclear; FBI agents and administrators are secretive about the bureau's work with informants. However, a senior FBI official indicated that bureau rules designed to prevent serious crimes by informants may not always be followed by agents in the field.

The nine former FBI agents spoke -- on the record -- not to criticize the practice of overlooking violent crimes by informants, but rather to defend it as a necessary evil of criminal investigation.

"The bureau has to encourage these guys to be themselves and do what they do," said Joseph O'Brien, a former FBI informant coordinator in New York City who retired in 1991. "If they stop just because they are working with the FBI, somebody's going to question them."

Gary Penrith, who retired in 1992 after a career that included serving as the bureau's deputy assistant director of intelligence, added: "Every one of the good ones are outlaws."

The former agents said it makes sense to overlook an informant's involvement in robberies or beatings if the information he is providing helps solve or prevent worse crimes. But sometimes, they added, even homicides were ignored.

Several said they would never protect known killers, but others said it was defensible in some circumstances.

"You have to weigh the odds of whether killing one or two people is better than killing a whole planeload," said M. Wesley Swearingen, whose service as an agent from 1959 to 1977 included tours in Los Angeles and Chicago.

For example, he said, agents ignored the killing of a small-time mobster by an FBI informant in Chicago in the 1960s because "the information that the FBI was getting was more important. Somebody in the mob is going to kill that person anyway."

The former agents interviewed were generally more forthcoming about their FBI experiences than the bureau might like. Four have written books that sometimes diverge from the official line, and O'Brien resigned from the agency in a dispute over his book's contents.

However, the former agents remained faithful to the bureau's policy of protecting informant identities, declining to name even those who have killed.

A review of court cases and published accounts identified 11 informants who are known to have killed while working with the agency or to have been shielded by their bureau handlers from prosecution for homicides committed before they were recruited.

Those 11, including three mobsters involved in the Boston scandal, are believed to have killed at least 52 people between the 1960s and the mid-1990s.

Previously, these cases had been reported as isolated incidents, but in light of the interviews with former agents, they appear to be a part of a wider pattern.

Clifford Zimmerman, a Northwestern University law professor who studies informant practices, said it is immoral, and perhaps illegal, for agents to shrug off violent crimes.

"They're doing their own little cost-benefit analysis and really not taking into account, in my opinion, the damage to society that these people are causing," he said. "Is a federal official entitled to make that decision -- that one person's life is more valuable than another's?"

Sometimes it amounts to that, former agents acknowledge.

"What it comes down to is who's got the best information," said Robert Fitzpatrick, assistant director of the Boston field office when he retired in 1986. Informants who provided valuable information in major mob investigations "generally would be savable" even if they killed, he said.

Several former agents expressed sympathy for John Connolly, the former Boston agent sentenced in September to 10 years in prison for his role in protecting two organized-crime kingpins, James "Whitey" Bulger and Stephen "The Rifleman" Flemmi. The two are accused in 18 homicides, including 11 committed while they were serving as FBI informants.

The bureau's rules for handling informants specifically ban what Connolly did. The rules, in effect for the past 26 years, forbid informants from participating in violent crimes. Officially, informants are allowed only nonviolent crimes, and these only when authorized as necessary to keep the informants in a position to supply information.

Connolly is by no means the only agent who has bent or broken these rules, the retired agents said.

"I'd be the first to tell you that agents who were doing this [handling informants] every day pushed the envelope," said Dennis O'Callaghan, a former chief of the organized-crime unit who retired from the bureau in 1991.

Joseph R. Lewis, a deputy assistant FBI director in charge of criminal investigations and intelligence, said he is "fairly confident" that most field agents follow the rules. However, he added in a recent interview at FBI headquarters, "it probably happens" that some agents shut their eyes to unauthorized crimes committed by valued informants.

Informants can be difficult to control, Lewis said. "The good ones are con artists, and they're going to try to get something over on you."

Consider the case of Gregory Scarpa Sr. In the 1990s, while informing on the mob for the FBI, he also participated in gang warfare for control of New York City's Colombo crime family, killing as many as 13 rivals.

Senior FBI officials knew that Scarpa was suspected of murder but let him keep working as an informant, Lindley DeVecchio, Scarpa's bureau handler, later testified in court.

Scarpa subsequently admitted to a role in three of the killings and pleaded guilty to murder in 1993. In prison, he died of complications of AIDS contracted from a blood transfusion.

Murder provided Scarpa with a great cover for his work with the FBI, said Alan S. Futerfas, a lawyer who defended some of the mobsters Scarpa informed on. "People thought there was no way in the world that the FBI would let this guy run around killing people."

Most of those Scarpa had informed on were his gangland enemies. Informants commonly inform to eliminate rivals or settle grudges, the retired agents said. Just as the FBI uses them, they use the FBI, former agents said.

"Do agents know they're not telling them everything? Absolutely," said O'Callaghan. He called it a "don't ask, don't tell situation."

Agents avoid asking -- and if possible avoid hearing -- anything that would incriminate their informants in violent crimes, several agents said.

"You don't really want to know," O'Brien said, because if an agent doesn't know, he isn't breaking the rules.

William Turner, who worked in five field offices before retiring from the FBI in 1961, said he "kind of intimated" to his informants that they should keep their unauthorized crimes to themselves.

Occasionally, an informant might hint that he is planning a violent crime, hoping for an indication that the bureau will protect him from prosecution, the former agents said. "I wouldn't listen to it," said Joseph L. Schott, a former agent in New Jersey and Texas.

The retired agents defended these evasions as necessary to keep information flowing, but they expressed concern about the human cost when informants conclude they can break the law with impunity.

Valuable informants "know if they are cooperating with us, they're not necessarily going to be targeted for prosecution," said Joe Griffin, a former administrator in the bureau's organized-crime unit.

Turner added: "If I intimated to an informant that I didn't want to know about his own personal activity, for obvious reasons, that might be interpreted as a license to kill."

Lewis insisted that top bureau officials will not tolerate violent informants.

That's why field agents don't always tell Washington about them, Penrith said. "Do they always bring that to a supervisor? Hell, no, they don't," he said. And when he was supervising agents, he said, "I don't go and ask."

New England organized crime figure Stephen "The Rifleman" Flemmi worked off and on as an FBI informant from 1965 to 1990.Gregory Scarpa Sr., a Colombo family mob captain and a longtime FBI informant, appears at a New York news conference in 1992. Former Boston FBI agent John Connolly was sentenced to 10 years in prison for protecting former mob informants. Boston mobster and fugitive James "Whitey" Bulger provided information to the FBI about Boston's Angiulo crime family.