Craig Monteilh describes how he pretended to be a radical Muslim in order to root out potential threats, shining a light on some of the bureau's more ethically murky practices

The ex-FBI informant with a change of heart: 'There is no real hunt. It's fixed'

Craig Monteilh says he did not balk when his FBI handlers gave him the OK to have sex with the Muslim women his undercover operation was targeting. Nor, at the time, did he shy away from recording their pillow talk.

"They said, if it would enhance the intelligence, go ahead and have sex. So I did," Monteilh told the Guardian as he described his year as a confidential FBI informant sent on a secret mission to infiltrate southern Californian mosques.

It is an astonishing admission that goes to the heart of the intelligence surveillance of Muslim communities in America in the years after 9/11. While police and FBI leaders have insisted they are acting to defend America from a terrorist attack, civil liberties groups have insisted they have repeatedly gone too far and treated an entire religious group as suspicious.

Monteilh was involved in one of the most controversial tactics: the use of "confidential informants" in so-called entrapment cases. This is when suspects carry out or plot fake terrorist "attacks" at the request or under the close supervision of an FBI undercover operation using secret informants. Often those informants have serious criminal records or are supplied with a financial motivation to net suspects.

In the case of the Newburgh Four – where four men were convicted for a fake terror attack on Jewish targets in the Bronx – a confidential informant offered $250,000, a free holiday and a car to one suspect for help with the attack.

In the case of the Fort Dix Five, which involved a fake plan to attack a New Jersey military base, one informant's criminal past included attempted murder, while another admitted in court at least two of the suspects later jailed for life had not known of any plot.

Such actions have led Muslim civil rights groups to wonder if their communities are being unfairly targeted in a spying game that is rigged against them. Monteilh says that is exactly what happens. "The way the FBI conducts their operations, It is all about entrapment … I know the game, I know the dynamics of it. It's such a joke, a real joke. There is no real hunt. It's fixed," he said.

But Monteilh has regrets now about his involvement in a scheme called Operation Flex. Sitting in the kitchen of his modest home in Irvine, near Los Angeles, Monteilh said the FBI should publicly apologise for his fruitless quest to root out Islamic radicals in Orange County, though he does not hold out much hope that will happen. "They don't have the humility to admit a mistake," he said.

Monteilh's story sounds like something out of a pulp thriller. Under the supervision of two FBI agents the muscle-bound fitness instructor created a fictitious French-Syrian alter ego, called Farouk Aziz. In this disguise in 2006 Monteilh started hanging around mosques in Orange County – the long stretch of suburbia south of LA – and pretended to convert to Islam.

He was tasked with befriending Muslims and blanket recording their conversations. All this information was then fed back to the FBI who told Monteilh to act like a radical himself to lure out Islamist sympathizers.

Yet, far from succeeding, Monteilh eventually so unnerved Orange County's Muslim community that that they got a restraining order against him. In an ironic twist, they also reported Monteilh to the FBI: unaware he was in fact working undercover for the agency.

Monteilh does not look like a spy. He is massively well built, but soft-spoken and friendly. He is 49 but looks younger. He lives in a small rented home in Irvine that blends into the suburban sprawl of southern California. Yet Monteilh knows the spying game intimately well.

By his own account Monteilh got into undercover work after meeting a group of off-duty cops working out in a gym. Monteilh told them he had spent time in prison in Chino, serving time for passing fraudulent checks.

It is a criminal past he explains by saying he was traumatised by a nasty divorce. "It was a bad time in my life," he said. He and the cops got to talking about the criminals Monteilh had met while in Chino. The information was so useful that Monteilh says he began to work on undercover drug and organised crime cases.

Eventually he asked to work on counter-terrorism and was passed on to two FBI handlers, called Kevin Armstrong and Paul Allen. These two agents had a mission and an alias ready-made for him.

Posing as Farouk Aziz he would infiltrate local mosques and Islamic groups around Orange County. "Paul Allen said: 'Craig, you are going to be our computer worm. Our guy that gives us the real pulse of the Muslim community in America'," Monteilh said.

The operation began simply enough. Monteilh started hanging out at mosques, posing as Aziz, and explaining he wanted to learn more about religion. In July, 2006, at the Islamic Center of Irvine, he converted to Islam.

Monteilh also began attending other mosques, including the Orange County Islamic Foundation. Monteilh began circulating endlessly from mosque to mosque, spending long days in prayer or reading books or just hanging out in order to get as many people as possible to talk to him.

"Slowly I began to wear the robes, the hat, the scarf and they saw me slowly transform and growing a beard. At that point, about three or four months later, [my FBI handlers] said: 'OK, now start to ask questions'."

Those questions were aimed at rooting out radicals. Monteilh would talk of his curiosity over the concepts of jihad and what Muslims should do about injustices in the world, especially where it pertained to American foreign policy.

He talked of access to weapons, a possible desire to be a martyr and inquired after like-minded souls. It was all aimed at trapping people in condemning statements. "The skill is that I am going to get you to say something. I am cornering you to say "jihad"," he said.

Of course, the chats were recorded.

In scenes out of a James Bond movie, Monteilh said he sometimes wore a secret video recorder sewn into his shirt. At other times he activated an audio recorder on his key rings.

Monteilh left his keys in offices and rooms in the mosques that he attended in the hope of recording conversations that took place when he was not there. He did it so often that he earned a reputation with other worshippers for being careless with his keys. The recordings were passed back to his FBI handlers at least once a week.

He also met with them every two months at a hotel room in nearby Anaheim for a more intense debriefing. Monteilh says he was grilled on specific individuals and asked to view charts showing networks of relationships among Orange County's Muslim population.

He said the FBI had two basic aims. Firstly, they aimed to uncover potential militants. Secondly, they could also use any information Monteilh discovered – like an affair or someone being gay – to turn targeted people into becoming FBI informants themselves.

None of it seemed to unnerve his FBI bosses, not even when he carried out a suggestion to begin seducing Muslim women and recording them.

At one hotel meeting, agent Kevin Armstrong explained the FBI attitude towards the immense breadth of Operation Flex – and any concerns over civil rights – by saying simply: "Kevin is God."

Monteilh's own attitude evolved into something very similar. "I was untouchable. I am a felon, I am on probation and the police cannot arrest me. How empowering is that? It is very empowering. You began to have a certain arrogance about it. It is almost taunting. They told me: 'You are an untouchable'," he said.

But it was not always easy. "I started at 4am. I ended at 9.30pm. Really, it was a lot of work … Farouk took over. Craig did not exist," he said. But it was also well paid: at the peak of Operation Flex, Monteilh was earning more than $11,000 a month.

But he was wrong about being untouchable.

Far from uncovering radical terror networks, Monteilh ended up traumatising the community he was sent into. Instead of embracing calls for jihad or his questions about suicide bombers or his claims to have access to weapons, Monteilh was instead reported to the FBI as a potentially dangerous extremist.

A restraining order was also taken out against him in June 2007, asking him to stay away from the Islamic Center of Irvine. Operation Flex was a bust and Monteilh had to kill off his life as Farouk Aziz.

But the story did not end there. In circumstances that remain murky Monteilh then sued the FBI over his treatment, claiming that they abandoned him once the operation was over.

He also ended up in jail after Irvine police prosecuted him for defrauding two women, including a former girlfriend, as part of an illegal trade in human growth hormone at fitness clubs. (Monteilh claims those actions were carried out as part of another secret string operation for which he was forced to carry the can.)

What is not in doubt is that Monteilh's identity later became public. In 2009 the FBI brought a case against Ahmad Niazi, an Afghan immigrant in Orange County.

The evidence included secret recordings and even calling Osama bin Laden "an angel". That was Monteilh's work and he outed himself to the press to the shock of the very Muslims he had been spying on who now realised that Farouk Aziz – the radical they had reported to the FBI two years earlier – had in fact been an undercover FBI operative.

Now Monteilh says he set Niazi up and the FBI was trying to blackmail the Afghani into being an informant. "I built the whole relationship with Niazi. Through my coercion we talked about jihad a lot," he said. The FBI's charges against Niazi were indeed later dropped.

Now Monteilh has joined an American Civil Liberties Union lawsuit against the FBI. Amazingly, after first befriending Muslim leaders in Orange County as Farouk Aziz, then betraying them as Craig Monteilh, he has now joined forces with them again to campaign for their civil liberties.

That has now put Monteilh's testimony about his year undercover is at the heart of a fresh legal effort to prove that the FBI operation in Orange County unfairly targeted a vulnerable Muslim community, trampling on civil rights in the name of national security.

The FBI did not respond to a request from the Guardian for comment.

It is not the first time Monteilh has shifted his stance. In the ACLU case Monteilh is now posing as the sorrowful informant who saw the error of his ways.

But in previous court papers filed against the Irvine Police and the FBI, Monteilh's lawyers portrayed him as the loyal intelligence asset who did sterling work tackling the forces of Islamic radicalism and was let down by his superiors.

In those papers Monteilh complained that FBI agents did not act speedily enough on a tip he gave them about a possible sighting of bomb-making materials. Now Monteilh says that tip was not credible.

Either way it does add up to a story that shifts with the telling. But that fact alone goes to the heart of the FBI's use of such confidential informants in investigating Muslim communities.

FBI operatives with profiles similar to Monteilh's – of a lengthy criminal record, desire for cash and a flexibility with the truth – have led to high profile cases of alleged entrapment that have shocked civil rights groups across America.

In most cases the informants have won their prosecutions and simply disappeared. Monteilh is the only one speaking out. But whatever the reality of his year undercover, Monteilh is almost certainly right about one impact of Operation Flex and the exposure of his undercover activities: "Because of this the Muslim community will never trust the FBI again."