The beheading of an American journalist by an extremist with a London accent is prompting deep reckoning among Britons over the particularly vicious role their countrymen are playing in the destabilisation of the Middle East.
Security officials in London have been sounding the alarm for more than a year over the large number of foreigners in Syria, with the chief of Scotland Yard telling reporters last week that about 500 Britons are among the thousands of Westerners who have joined the fight.
The government's concerns have focused on the possibility that some of those fighters will return home, newly radicalised, and carry out attacks in Britain.
But the video released Tuesday showing the execution of American journalist James Foley highlighted just how central foreigners have become to some of the most extreme behaviour by militant Islamist groups in Iraq and Syria.
"Foreign fighters are often used for the most brutal acts because they are the most ideologically motivated," said Peter Neumann, director of the International Center for the Study of Radicalisation. "The locals may say, 'That's not the kind of thing we do here.' But the outsiders don't know that."
Security officials say that most of the Britons who have left to fight in Syria and increasingly in Iraq have joined the Islamic State, the extremist group that asserted responsibility for Foley's killing.
Neumann said the group probably selected a Brit to carry out the execution because he was willing and it knew that his voice would resonate across the West.
"They probably picked the Brit because there was no American," Neumann said, noting that only about 100 Americans have joined the fight in Syria. "It was important for them to have someone who speaks English fluently."
Before travelling to Syria, most British recruits will have watched numerous videos of beheadings and suicide bombings, many of them carried out by their fellow Europeans, Neumann said.
The video's release came amid anguished debates here over Britain's role in the Iraq and Syria wars and in confronting homegrown extremism.
Prime Minister David Cameron, who had been on vacation with his family, rushed back to London on Wednesday morning to chair emergency meetings at 10 Downing Street. On Twitter, he called Foley's killing "shocking and depraved."
Cameron later told the BBC that British investigators were still trying to identify the executioner but that "it looks increasingly likely that it is a British citizen". Experts on British dialects said his pronunciation suggested he was from London.
Although the killer was masked, many of the Britons who travel to Syria to fight are well known to investigators because they do little to hide their identities on social media, posting videos, photos and tweets that testify to their acts of extreme violence.
"We're absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities," British Foreign Secretary Philip Hammond told the BBC. "Many of these people may seek at some point to return to the UK, and they would then pose a direct threat to our domestic security."
Hammond said the government would consider sending British troops to train Iraqi security forces but that they would have no combat role. Until now, Britain's response to the growing violence in Iraq has been limited to humanitarian aid, surveillance and distributing supplies to Kurdish fighters battling the Islamic State. It has not joined with the United States in carrying out airstrikes.
But in a sign of the growing European appetite for action, traditionally non-interventionist Germany said Wednesday that it was prepared to send weapons to the Kurds. French President François Hollande, who has been among the most hawkish of the European leaders, called for an international conference to coordinate opposition to the Islamic State.
"We have to come up with a global strategy to fight this group, which is structured, has significant weapons, and threatens countries like Iraq, Syria and Lebanon," Hollande said.
The call came amid growing concern that Europe lacks a clear strategy to combat the threat either on the continent or in the Middle East.
The Church of England recently wrote to Cameron asserting that "we do not seem to have a coherent or comprehensive approach to Islamist extremism as it is developing across the globe."
The sense that British security services may be losing their grip on the problem of homegrown extremism has become especially acute in recent days as Islamic State supporters have openly distributed leaflets in central London heralding the return of the caliphate.
Anjem Choudary, a radical London preacher who has claimed that the leaflets are the work of his students, said in an interview that there was "a reawakening of Muslims in Britain and Europe" and that he believes the commander of the Islamic State, Abu Bakr al-Baghdadi, is the rightful leader of Muslims worldwide.
Foley's killing, he said, can be understood as a response to American aggression against Muslims and what he described as the Western media's inaccurate coverage of conflicts in the Middle East.
"We have seen that Western media has not really portrayed the reality going on in Muslims lands, the suffering of Gaza," Choudary said.
That logic helps explain why security services are thought to be keeping a close watch on Chaudary and his followers. But officials have acknowledged that they are struggling to keep tabs on about 250 Britons who have gone to Syria and come back despite recent warnings from the government that they will be arrested upon their return.
The Foley video's message of war on the West is likely to intensify anxiety here that one or more of those returnees will try to pull off an attack in Europe.
"The fear is that this is likely to escalate," said Ghaffar Hussain, managing director of the Quilliam Foundation, a London-based anti-extremism organisation. "They will do their best to show that they can get at Western targets."
Hussain said groups such as the Islamic State prey on the "large number of second-generation Muslims here who are disillusioned with Britain and confused about their identity. It's a generation of people who are susceptible to extremist initiatives."
And for all the government's efforts, Hussain said, much more work is needed to dissuade young Muslims from joining the fight and to counter extremist narratives.
"I don't think Britain has come to terms with the fact that it's dealing with a very large-scale problem," he said.