Last night, chief foreign affairs correspondent Margaret Warner reported from London about what's driving so many British and European Muslims to travel to Syria to fight with the Islamic State group.

　　Tonight, she reports on what the British government and community leaders are trying to do to stop that trend.

　　MARGARET WARNER: Imran Khawaja was supposed to be a dead man. The Londoner went to Syria last January to join a radical group affiliated with Islamic State. He was a star of its online recruitment videos, usually masked, brandishing weapons.

　　Then, last June, the group announced on social media that he'd been killed in battle. But it was all a ruse. That same day, the 27-year-old was arrested trying to sneak back through the British port of Dover. Last week, he pled guilty to four terrorism-related crimes, which could carry life in prison.

　　Just days ago, Scotland Yard announced it had made 165 Syria-related terrorist arrests in 2014, including Khawaja's, a six-fold increase over the 25 arrested the previous year.

　　MARK ROWLEY, Assistant Commissioner for Specialist Crime and Operations, Metropolitan Police Department, London: The surge of work we have seen over the last year is stretching us.

　　MARGARET WARNER: Britain's counterterrorism chief, Mark Rowley, says not even the U.K.'s 40 years of dealing with Irish Republican-inspired terrorism prepared them for the scope of the new threat from Islamic extremists going to and returning from Syria and Iraq.

　　MARK ROWLEY: Half of the people who we are concerned about who traveled to Syria weren't previously on our radar, so new people are being drawn into this. This isn't the usual suspects, to use an old phrase. Some of this is new people coming into the terrorist cause.

　　MARGARET WARNER: To date, authorities estimate 600 U.K. Muslims have gone to join jihadi groups in Syria, and nearly half may have returned.

　　None of these returnees has pulled off an attack in the U.K., but authorities say they foiled five major plots last year that would have lost many lives. This month's attacks in Paris also highlighted the threat posed throughout Western Europe by under-the-radar jihadis trained abroad.

　　Rowley said authorities are trying to stem the surge of these fighters at every point in the pipeline, depending on tips from the Muslim community, which have surged in the past year.

　　MOHAMMED KOZBAR, Chairman, Finsbury Park Mosque: “You scum — you scum should be killed. Muslim, rot in hell.”

　　MARGARET WARNER: Mohammed Kozbar, chairman of the Finsbury Park Mosque, who is trying to help, read us anonymous hate mail received after Paris.

　　His London mosque was once a hotbed of extremist preaching, under Imam Abu Hamza al-Masri, convicted in New York this month for instigating terrorist attacks. Shoe bomber Richard Reid was among the radicals who prayed there. In 2005, Kozbar and others wrested control of the mosque away from its radical leaders.

　　Now, he says, they work with members, starting at the youngest age, warning about the dangers of extremism.

　　MOHAMMED KOZBAR: We look to people who are vulnerable, especially young people, who we see that they might be in a way driven away from the mosque to extremism. So we try to engage with these people in advance.

　　MARGARET WARNER: That's the idea behind the Active Change Foundation in East London's Leyton neighborhood. It's the brainchild of Hanif Qadir, who went to Afghanistan to fight against the U.S. after 9/11, but left when he saw cruelty being committed by both sides.

　　HANIF QADIR, CEO, Active Change Foundation: I got caught up in a network of individuals afterwards known to be from al-Qaida. It's a classic case of being recruited into a network and being radicalized.

　　MARGARET WARNER: Qadir returned to London on a mission to prevent violent extremism among its Muslims. At first, his message fell on deaf ears.

　　HANIF QADIR: Nobody was appreciating the fact that, you know, we have got problems in our community. And then we had 7/7. At that point, it was like, well, we told you so, but now we hope that you can understand and help us to get on with the work that needs to be done.

　　MARGARET WARNER: The London transport suicide bombings of July 2005, which killed more than 50 civilians, were the work of four British-born young Muslims.

　　The attacks sparked new laws and programs to combat extremist terrorism; 18-year-old Javid Khan, who moved to London from Afghanistan with his family in 2010, said Qadir's program helped him as a teenager resist radicalizing influences.

　　JAVID KHAN, United Kingdom: This is the only place that you can find out about what's going on in the world and how we can avoid recruiters of ISIS and other extremist groups. I don't want that name on me or on my family.

　　HAMZA ABDULWAHI, United Kingdom: This is a highly-populated area full of Muslims, so there was a high chance of me meeting the wrong type of people.

　　MARGARET WARNER: Twenty-two-year old university student Hamza Abdulwahi, who moved to the neighborhood at 13, says the program helped him understand that the way of Allah doesn't include violence.

　　HAMZA ABDULWAHI: If the person went there to fight, you have to be concerned, because, clearly, he's not of the right mind. If he went for other motives, to go kill people, and that person comes back, he could clearly do the same thing here.

　　MARGARET WARNER: But there is a fierce debate here about the effectiveness of these programs, whether it's intervening before someone goes to fight or trying to rehabilitate them afterwards, concedes the minister in charge for the U.K. Home Office, James Brokenshire.

　　Britain does have one of the highest percentages of Muslims youth going to fight. What is your evidence that your programs are successful?

　　JAMES BROKENSHIRE, Minister for Security and Immigration, Home Office, United Kingdom: We have had around 2,000 referrals, and several hundred people are receiving direct support, in other, words to challenge the ideology. But I think it's — there's no one size fits all. So, it is a complex picture, one that we are vigilant on and are constantly challenging ourselves as to what more that we can do.

　　MARGARET WARNER: But there is criticism from the Muslim community that these programs target only Muslims.

　　MOAZZAM BEGG, Outreach Director, CAGE: The basic programs like Prevent, for example, Preventing Violent Extremism, as it was called in the beginning, wanted communities literally to spy on one another.

　　MARGARET WARNER: Moazzam Begg, detained at Guantanamo for nearly three years on charges of attending al-Qaida training camps in Afghanistan, returned to London in 2005 to found the group CAGE. It opposes what it sees as draconian anti-terrorism measures.

　　Last year, Begg was arrested for going to Syria in 2012 and 2013 to train rebels fighting President Bashar Assad's forces. Western governments were supporting some anti-Assad fighters then too. The charges were later dropped.

　　So what does happen when they come home?

　　MOAZZAM BEGG: I was imprisoned with many of these young men who have returned. People don't know whether they have committed a crime or not. They went for benign reasons. They thought they were helping the Syrians, and they found out something else was going on.

　　MARGARET WARNER: But counterterrorism chief Rowley is skeptical of such claims.

　　MARK ROWLEY: Some of it sounds a bit incredible to me, because they will say, well, my son's been out there, he regrets what he's done, he wants to come back, he's sorry.

　　If someone has traveled and not got involved with anything, then we can — we can help them. But people who are going out there, they are planning to join a terrorist group, you can't possibly not realize how awful the activities out there and that ISIS are all regarded by all the Western world as a terrorist group. If you are going to take part in that, then we are going to investigate you and we're going to throw the book at you.

　　THERESA MAY, Home Secretary, United Kingdom: Quite simply, Mr. Speaker, if we want the police and the security services to protect the public and save lives, they need this capability.

　　MARGARET WARNER: Britain is looking to add pages to that book. A controversial new anti-terror bill is working its way through Parliament. Critics dub it the snoopers bill. It would increase government's powers to monitor suspected extremists and expand the universe of people asked to report suspected cases to authorities.

　　All this, says Finsbury Park Mosque chairman Kozbar, will put British Muslims even more in the crosshairs.

　　MOHAMMED KOZBAR: To tell the Muslim community that you have to spy on your children, to tell them that when you see something wrong or you think that there is something wrong, you have to report it and all of this, this is not helpful. We want, as British people, to be safe and to be secure, but we want to do it the right way.

　　MARGARET WARNER: For now, though, the right way to counter the threat remains in dispute.

　　I'm Margaret Warner in London for the NewsHour.