"This is Farouk al-Aziz, code name Oracle," he said into the keys as he sat in his parked car in this quiet community south of Los Angeles. "It's November 13th, 4:30 a.m. And we're hot."

The undercover FBI informant - a convicted forger named Craig Monteilh - then drove off for 5 a.m. prayers at the Islamic Center of Irvine, where he says he spied on dozens of worshipers in a quest for potential terrorists.

Since the 2001 terrorist attacks, the FBI has used informants successfully as one of many tactics to prevent another strike in the United States. Agency officials say they are careful not to violate civil liberties and do not target Muslims.

But the FBI's approach has come under fire from some Muslims, criticism that surfaced again late last month after agents arrested an Oregon man they said tried to detonate a bomb at a Christmas tree-lighting ceremony. FBI technicians had supplied the device.

In the Irvine case, Monteilh's mission as an informant backfired. Muslims were so alarmed by his talk of violent jihad that they obtained a restraining order against him.

He had helped build a terrorism-related case against a mosque member, but that also collapsed. The Justice Department recently took the extraordinary step of dropping charges against the worshiper, who Monteilh had caught on tape agreeing to blow up buildings, law enforcement officials said. Prosecutors had portrayed the man as a dire threat.

Compounding the damage, Monteilh has gone public, revealing secret FBI methods and charging that his "handlers" trained him to entrap Muslims as he infiltrated their mosques, homes and businesses. He is now suing the FBI.

Officials declined to comment on specific details of Monteilh's tale but confirm that he was a paid FBI informant. Court records and interviews corroborate not only that Monteilh worked for the FBI - he says he made $177,000, tax-free, in 15 months - but that he provided vital information on a number of cases....