FBI agents often violate bureau's rules for handling confidential informants that were revised after FBI abuses in the 1990s, the Justice Department's internal watchdog said Monday.

A review of 120 confidential informant files from FBI offices around the country found violations in 104 cases, or 87 percent, Inspector General Glenn A. Fine said. His 301-page report, parts of which were blacked out, examined FBI compliance with rules that govern most criminal investigations.

The report said agents failed to assess informants' suitability or get permission for informants to engage in activity that otherwise would be illegal. Agents did not convey proper instructions or tell prosecutors when informants had committed crimes that were not authorized by their FBI handlers, Fine said.

FBI Director Robert Mueller (search), according to the report, said many agents find the required paperwork cumbersome. In a statement, the FBI said the violations were "administrative" and that officials have begun simplifying procedures.

Among the material in the report that was withheld from the public is how many confidential informants the FBI uses. Some have provided information to the FBI for more than six years. Others are designated "high-level" informants because they are part of the senior leadership of a group the FBI is investigating.

A third category of informants identified in the report includes lawyers, doctors and clergy -- all obligated to keep confidential their dealings with clients -- or members of the news media.

FBI spokesman Ed Cogswell said the bureau would not identify whether any of its informants are clergy or members of the media.

"We can use them and if a need presents, I'm sure we do," Cogswell said. "But we do not want to identify what our sources are."

Confidential informants are critical to some prosecutions, the report said. But their use by the FBI also "presents serious risks, including the risk that informants may claim that their criminal activities were authorized or acquiesced in by the government," Fine said.

The Attorney General's Investigative Guidelines were developed in the mid-1970s in response to FBI surveillance and infiltration of civil rights and other protest groups in the 1950s and 1960s.

The section on informants was overhauled in 2001, after celebrated cases in which FBI agents protected mobsters from prosecution or tipped them off to investigations while simultaneously using them as FBI informants.

In one case, former FBI agent John J. Connolly Jr. tipped off Boston mobster James "Whitey" Bulger to a looming racketeering indictment, causing Bulger to flee. He remains at large.

The FBI has said informants are important to counterterrorism and criminal investigations.

Two informants helped lure Yemeni Sheik Mohammed Ali Hassan al-Moayad and an aide to a meeting in which the cleric was secretly recorded promising to funnel money to Hamas and al-Qaida. Al-Moayad was sentenced to 75 years in prison. Mohammed Mohsen Yahya Zayed received a 45-year sentence.

The case, however, was nearly derailed when one informant, who was the government's star witness, set himself on fire in Washington in what he later described as an attempt to get more money from the FBI, which paid him at least $100,000.

Mohamed Alanssi recovered in time for the trial.