The biggest looming security threat for many Western countries, including the United States, will come from hundreds of jihadists returning home after cutting their teeth fighting in the Syrian civil war, say European officials and terrorism experts, who point to a suicide bombing carried out in Aleppo last week by a British volunteer as a disturbing new development.

Another departure sending shock waves through the British government came last night when Britain’s Sky News broadcast social media footage from a British Facebook account showing British jihadists allegedly engaging in the torture of a moderate rebel, who was beaten so mercilessly that his tormentors broke a metal rod in half. The moderate fighter was accused of insulting Allah and pleaded to his captors that he, too, was a Muslim.

Last year, video footage of Belgian jihadists allegedly showed them involved in beheadings.

“We are seeing European fighters going out there and becoming really involved in some of the more brutal elements. Of course, the big concern is what happens with these individuals when they come back home—some of them are likely to die on the battlefield but some will survive. Some will return,” worries counterterrorism expert Raffaello Pantucci of the London-based Royal United Service Institute.

The Aleppo bombing on February 6—believed to be the first undertaken by a British jihadist in the Syrian conflict—targeted a prison in the northern Syrian city and the explosion allowed about 300 prisoners to escape the government-controlled facility. The truck used by the British jihadist—he used the nom de guerre of Abu Suleiman al Britani and is thought to have come from London—sported the flags of the al-Qaeda affiliate Jabhat al Nusra, the most prolific and proficient militant group when it comes to suicide bombings in the Syrian war.

Al-Britani’s death and nationality was established by Shiraz Maher, a senior research fellow at the International Centre for the Study of Radicalization ((ISCR) at Kings College in London, who has been closely monitoring the flow of young foreigners enlisting with jihadist groups in Syria.

Ten Britons have been killed fighting in Syria, according to monitoring groups, and the head of Britain’s foreign intelligence wing MI6, John Sawers, told a British parliamentary panel in December that 300 young British Muslims had gone off to join jihadist groups in Syria—a bigger number than joined up for the wars in Afghanistan and Iraq combined.

The death of one of the British volunteers, Aby Layth, who was killed in rebel infighting in the border town of al-Rai, was celebrated on jihadist social media forums with pleas that Allah accept him into paradise as a reward. The dead jihadist is thought to have had ties to the United States as well, say British officials.

The stream of British fighters has prompted the British government to consider stripping UK citizenship from any dual-national who has fought in Syria. And a British government official told the Daily Beast that a multiagency review is being conducted currently to see if any returning fighters can be prosecuted for their activities in Syria.

Britain isn’t the only Western country seeing young Muslims heading to Syria and learning how to fight and being trained as terrorists. The Washington DC-based research group the Middle East Media Research Institute (MEMRI) recently picked up the social media postings of a 25-year-old who claims to be from Chicago and who says he has joined the Islamic State of Iraq and Sham (ISIS), the group recently disavowed by al-Qaeda’s top leadership because of the waywardness of its leader.

In his postings on Facebook and Ask.fm, a Latvian-based social networking website, “AT” urges other young Americans to join the jihad. He advises young recruits to “bring thermals, base-layers, Goretex, warm socks,” saying he picked up what he needed shopping at Walmart, whose corporate marketing slogan ironically is “Save Money. Live Better.”

AT isn’t the only Western jihadist using Ask.fm to market the jihadist message and tout the virtues and adventure of holy war, along with answering questions from would-be fighters thinking of going to Syria. Many of the foreign fighters are exploiting social media avenues to assist recruitment in a move that experts think has the blessing of jihadist leaders.

Two British jihadists who claim to currently be in Syria—Mahdi Hassan, a former Catholic private schoolboy, and Muhammad Hamidur Rahman, a onetime shop worker—also interact with sympathizers on Ask.fm. In one online conversation Hassan says he misses his family but says, “what motivates me is that the Muslims of sham [Syria] are being killed, tortured and raped and as Muslims we must believe that they are also family.”

EU officials estimate that about 20 percent of the foreign fighters in Syria are from Western Europe, with France accounting for the largest group (about 700 is the estimate) followed by Britain and Germany. ISCR estimates up to 2,000 fighters from Europe are currently in Syria. Young Muslims from Denmark, Norway, Belgium and Austria have also gone to fight in the civil war.

“We have heard about Americans, Canadians and others, too. I really couldn’t put a number on it. And I don’t know anyone who can—credibly, that is,” says Jonathan Schanzer, a Mideast expert with the Foundation for Defense of Democracies, a Washington DC-based think tank.

He adds: “The threat of returnees is a big one. We learned that after the mujahedeen phenomenon in the 1980s. Fighters came back to their countries of origin and created new al-Qaeda affiliate groups. These were smaller jihad groups with local aims but which shared the global vision of al-Qaeda. Many of them are still around today—or their descendants are.”

EU interior ministers agreed at a recent meeting in Athens to explore ways of preventing young Europeans from leaving for Syria and to monitor the returnees. Last week, nearly 200 terrorism experts and official representatives from two dozen European cities met with EU authorities at The Hague to discuss strategies to counteract jihadist propaganda and develop de-radicalization programs.

In December, Turkish authorities claimed to European ambassadors in Ankara that they had it had deported 1,100 European citizens who travelled to Turkey to join al-Qaeda groups in Syria. The claim was made in response to European criticism that Turkey wasn’t doing enough to help the stem the stream of would-be European jihadists. The Turkish foreign minister insisted there are limits to what Turkey can do, asking mockingly, “Are we meant to stop anyone with a beard?”

Says Schanzer, “Turkey has not traditionally been a country that sets off alarm bells at U.S. immigration. But now it will. This is one of the consequences of Turkey’s lax border policies, not to mention its rather open support for salafi and jihad groups fighting across the border.”

Paul Pillar, a former national intelligence officer for the Near East at the US National Intelligence Council says returning fighters will need to be watched closely. “With returning jihadis the targeting task will in a sense be easier because they have already demonstrated through their actions that they are worth monitoring--it is not just a question of trying to figure out intentions or orientation from some suspicious remarks.”

Now a professor at Georgetown University, Pillar said Western countries need also to assess whether any home country laws can be applied. “In some cases it may be possible to determine that home country laws have been broken, in which case a criminal prosecution may be in order. In the United States, this could involve the law making it a crime to provide material support to a terrorist group.”