Hanif Qadir, a former extremist, runs the Active Change Foundation, a deradicalization project in London that works with young people at risk of embracing terrorism. (Lefteris Pitarakis/Associated Press)

The London-accented militant who delivered blood-curdling threats to the West before apparently beheading two American journalists has become, for most Britons, the masked face of foreign fighters in Syria.

But more typical, experts say, may be the Briton who recently called home from the front lines to say he’s fed up.

“The whole jihad was turned upside down,” the militant recently told Shiraz Maher, a senior researcher for the International Center for the Study of Radicalization at King’s College London. “Muslims are fighting Muslims. I didn’t come for that.”

The fighter’s disillusionment, experts say, has become a recurring theme among some of the thousands of young men and women from around the globe who have answered the Islamic State’s call for holy war but have found that the reality is significantly less glorious than what they were promised.

For those trying to stanch the flow of fighters and to combat extremism here in Britain, it is a perspective that could be the perfect antidote to Islamic State propaganda. And yet it is one that is seldom if ever heard here, in part because of government policy that focuses on keeping Britons who have gone to war from returning home — and locking them up if they even try.

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“A lot of them feel trapped by the Islamic State not letting them go and by the British government not letting them back,” said Richard Barrett, a former counterterrorism director with Britain’s foreign intelligence service, MI6. “But if you want people to understand that it’s bloody terrible out there, you have to hear from these people.”

The government has good reason to be extremely wary of allowing former fighters to come home after war-zone experiences that have left many more radical than ever — and possibly determined to strike the West.

British Prime Minister David Cameron recently called the prospect that they could return and carry out attacks here “a greater and deeper threat to our security than we have known before.” On the same day, British security services raised the terror threat level to “severe,” meaning an attack on British soil is now considered “highly likely.”

The government’s response has been to crack down hard on those suspected of planning to travel to Syria and on those who may already have been there. With the war in Syria little more than a budget-airline flight away, thousands of Europeans have been drawn in, including some 500 Britons. British police have arrested 69 people this year on suspicion of joining the fight.

To keep militants from slipping through, Cameron has sought to close loopholes in the law, including giving police the power to temporarily confiscate passports as fighters attempt to come and go at airports.

London Mayor Boris Johnson, touted as a possible future prime minister, has gone further, suggesting that suspected fighters be presumed guilty until proved innocent and should be stripped of their citizenship.

Such tough talk has dominated the discourse here, with little attention given to the idea of allowing some fighters to return and then funneling them through a comprehensive deradicalization program.

For many who understand the problem of homegrown extremism best, the current approach could be dangerously counterproductive.

“If you stop them from coming back, you’re going to create more grievances and more reasons for this country to be targeted,” said Hanif Qadir, chief executive of the Active Change Foundation (ACF), an anti-extremist group. “If we don’t leave a doorway open for them, they’re going to become more radicalized.”

Qadir knows the problem intimately. Appalled by reports of American airstrikes killing innocent civilians, he traveled to Pakistan and Afghanistan in 2002. He went with the intention of performing humanitarian work but said he was also attracted to the Taliban’s rhetoric of struggle against a foreign occupier and was prepared to fight alongside the insurgent group. Instead, he was repelled by what he found.

“If American soldiers were being hostile toward innocent civilians, so were al-Qaeda and the Taliban,” Qadir said. “This was hypocrisy.”

Qadir soon returned to Britain. The following year, he launched ACF to try to halt the spread of extremism in the northeast London neighborhood of Walthamstow, a community of neat, stucco-faced row houses where fish-and-chip shops dot the landscape alongside designer-hijab shops and travel agencies specializing in pilgrimages to Mecca.

Qadir said that the government should recognize that Britons who went to Syria to fight and became disillusioned while there can be some of the most effective spokesmen against the lure of the Islamic State.

“They’ve been sold a lie. They didn’t sign up for this sort of barbaric behavior, and now they want out,” Qadir said. “You can’t look at these individuals as potential threats. You have to look at them as potential assets.”

Radical groups in Syria have for years waged a slick campaign to recruit Westerners by appealing to their sense of religious duty and by assuring them that despite all evidence to the contrary, war isn’t so tough.

Recruiters have referred to the fight in Syria as “five-star jihad,” with impressionable would-be fighters promised ample accommodations, fine food, money and brides. Some recruitment materials have featuredsmiling fighters holding kittens.

But the reality is considerably less appealing, especially for Westerners who often lack any military training and may not even speak Arabic.

Experts say that foreign fighters are often given menial jobs far from the front lines, or are deemed expendable and used as suicide bombers. Many have been surprised to find that when they do fight, the battles are with fellow rebel groups, not with the government of Syrian President Bashar al-Assad.

The disillusioned fighter who recently contacted Maher, the King’s College researcher, said he is fighting in Syria alongside 30 Britons.

“Many people left to help the Syrian people. Then we got labeled as terrorists,” Maher quoted the militant as saying. “Now people want to come back, not to attack but because they found out jihad is not what they thought.”

Peter Neumann, a colleague of Maher’s at King’s College, said the government would be missing an opportunity if it did not allow some of the disillusioned fighters to return on the condition that they participate in a deradicalization program and submit to monitoring by the domestic intelligence agency, MI5.

“Clearly, people like that could become very powerful spokespeople that you could send into Muslim communities to speak out against [the Islamic State,]” he said. “All the government is currently offering is to say, ‘We're going to lock you up for 20 to 30 years,’ which is appropriate for very hardened extremists. But for the ones we are talking about, there should be another option.”

Karla Adam contributed to this report.