Opposition fighters in Syria prepare for battle north of Hama. As more Europeans flock to Syria to fight with the rebels, officials at home worry that they will return determined to stage attacks. (Ghaith Omran/AFP/Getty Images)

Word spread quickly last autumn in the rapidly gentrifying north London neighborhood where the Sebah brothers were raised: Mohamed and Akram had died in a car crash.

The news was devastating for friends and neighbors who had watched the brothers grow from affable and popular boys into promising young adults.

But the truth, as recently revealed in jihadist Web site postings, was darker still. Mohamed and Akram had been killed in Syria while fighting alongside rebel forces.

The Sebah brothers were part of a growing legion of Britons who have left behind their often comfortable lives here and joined an increasingly radicalized war effort — one that is just a short budget-airline flight away.

Dozens have been killed. Hundreds more remain on the battlefield. But most disconcerting for British security services are the ones — perhaps 50 or more — who are thought to have come home.

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British officials have expressed growing alarm in recent days over the possibility that returnees from the Syrian war, hardened and trained by their experiences in battle, will seek to carry out terrorist attacks. The head of Scotland Yard’s counterterrorism command said recently that it is “almost inevitable.”

That concern is matched by a fast-rising tally of arrests, with at least 14 Britons detained on charges related to travel to or from Syria this month, compared to a total of 24 last year.

Security officials say the several hundred Britons who are known to have joined the fight in Syria eclipses the totals for either Afghanistan or Iraq — two other conflicts that attracted radicalized young fighters from the West but that were more difficult to reach. They also acknowledge that there could be many more fighters who have slipped into Syria undetected, given the relative ease of travel by air to Turkey and then over land into the war zone.

The distress among security officials is pervasive in European capitals and in Washington. U.S. intelligence chief James R. Clapper Jr. told a congressional panel Wednesday that the Syrian war had attracted about 7,000 foreign fighters from as many as 50 nations and that at least one of the main jihadist groups in Syria aspires to carry out an attack in the United States.

But Europe is a far closer and more accessible target. The International Center for the Study of Radicalization, or ICSR, estimated last month that nearly 2,000 Western Europeans had traveled to Syria to fight and that the number was rising fast.

French officials say 700 came from France. French Interior Minister Manuel Valls asserted this month that returning fighters represent “the biggest threat that the country faces in the coming years.”

The anxiety has been especially acute in Britain, where memories are still fresh of the July 2005 transit bombings. Those attacks, which claimed 52 lives, were carried out by homegrown radicals, at least two of whom had received training in Pakistan.

“The penny hasn’t dropped. But Syria is a game-changer,” Richard Walton, who leads counterterrorism efforts at Scotland Yard, told the Evening Standard newspaper. “We are seeing it every day. You have hundreds of people going to Syria, and if they don’t get killed they get radicalized.”

Don’t bother coming back

Most of the Europeans who have gone to fight in Syria have joined not the moderate, Western-backed opposition — which is primarily Syrian and fighting the government of President Bashar al-Assad — but the extremist, al-Qaeda-affiliated groups that welcome foreigners, according to Shiraz Maher, a senior research fellow at the ICSR.

Maher said that Europeans who sign up for battle in Syria have a variety of motivations, from radical beliefs to humanitarian concern to boredom with life at home. But he said even those in the latter two categories risk being indoctrinated with a virulently anti-Western ideology once they reach the battlefield.

“If you stay on long enough, you imbibe the message,” he said.

And those Britons are not welcome home. Foreign Secretary William Hague told the BBC over the weekend that Britons should not travel to Syria for any reason and that those who have already gone there to fight should not bother returning if they want to avoid prosecution on terrorism charges.

Margaret Gilmore, a terrorism analyst with the Royal United Services Institute, said British anxiety is compounded by the fact that the government knows so little about what’s going on in Syria, especially compared with what it knew about Iraq and Afghanistan at the height of the wars there.

“A lot of these areas are a complete no man’s land, and the terrorist groups are able to operate and train to a high degree,” she said.

The radical groups, meanwhile, are using the lack of Western engagement in Syria to their advantage in recruiting Britons.

“The narrative that’s being spun by al-Qaeda-inspired groups is that it’s Muslim fighting Muslim, so the British government isn’t interested,” said Jonathan Russell, political liaison officer with the Quilliam Foundation, a London-based anti-radicalization group. “They say, ‘It’s your duty to look after your brothers and sisters in Syria because your so-called liberal and democratic government isn’t doing it for you.’ ”

‘The Lions of Britain’

It’s not known exactly what motivated the Sebah brothers to join the fight in Syria last summer. On jihadist Web sites, the pair have been lauded as “the Lions of Britain,” alongside a photo taken shortly before their death. In it, Mohamed, 28, and Akram, 24, smile as they clutch assault rifles.

In their exceptionally diverse north London neighborhood, where Somali hookah lounges nestle side by side with organic produce markets, friends and neighbors expressed astonishment that members of such a close-knit Eritrean family would leave to fight in such a distant war.

“They were magnificent guys. Very logical. Family people,” said a friend who declined to give his name because he didn’t want to attract the attention of authorities. “They would translate for people in the community who couldn’t speak English.”

Akram Sebah, the younger brother, had done particularly well — landing a plum job in a local real estate office.

Right around the corner was the Finsbury Park Mosque, which gained a reputation as a hub of Islamic radicalism in the late 1990s and early 2000s. The mosque has been under new leadership for the past decade, and although friends say that Akram Sebah attended prayers there, mosque director Mohammed Kozbar said he didn’t know either of the brothers.

But he said he understands what motivates young men to fight in Syria, even if he discourages them and tries to channel them toward humanitarian efforts.

“Every day they see women and children being massacred by the dictatorial regime of Assad. They feel they have to do something about it. And the fact that the international community has failed to end the suffering, this adds to their frustrations,” Kozbar said. “If you want to prevent these young people from going to Syria, the best way to do that is to stop the war.”

Karla Adam contributed to this report.