Prime Minister David Cameron said it seemed "increasingly likely" that US journalist James Foley was beheaded by a British jihadist, as experts sought to identify the English-accented executioner.
Cameron, who broke off his holidays for emergency meetings on the threat posed by Islamic State militants operating in Iraq and Syria who orchestrated Foley's execution, said the killer's probable nationality was "deeply shocking".
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Following talks with senior ministers at his Downing Street office in London, he said "far too many" Britons had travelled to the region to join jihadist groups and spelled out measures to combat extremism.
"Let me condemn the barbaric and brutal act that has taken place and let's be clear what this act is -- it is an act of murder, and murder without any justification," Cameron told reporters.
"We have not identified the individual responsible, but from what we have seen it looks increasingly likely that it is a British citizen.
"This is deeply shocking."
Scotland Yard, which leads counter-terror policing in Britain, confirmed it was investigating footage of the killing.
"The Metropolitan Police Counter Terrorism Command is investigating the contents of the video that was posted online" on Tuesday, it said.
The footage showed a masked militant with an English accent beheading Foley, who has been missing since he was seized in Syria in November 2012.
Some 400 to 500 British nationals are thought to have travelled to the region to link up with jihadists, and speculation is mounting that the executioner is one of these.
- 'Direct threat' to Britain -
"Far too many British citizens have travelled to Iraq and travelled to Syria to take part in extremism and violence," Cameron said, pledging to redouble efforts to stop people going.
He said Britain would remove the passports of suspect people contemplating travel, strip extremist material off the Internet and prosecute people participating in extremist violence.
Cameron also said London would work with the new Iraqi government, help arm the Kurdish forces fighting Islamic State militants in northern Iraq and use Britain's aid, diplomacy and "military prowess" to put pressure on "this appalling organisation".
However, he insisted there would be no "knee-jerk" escalation of British military involvement, saying the country was "not going to get involved in another Iraq war" while putting "combat boots on the ground" was "not something we should do".
Foreign Secretary Philip Hammond was among those attending the meetings Cameron chaired at his Downing Street office.
"We'll have to do some more analysis to make quite certain" of where the executioner came from, he told BBC radio.
He said the intelligence agencies were monitoring Britons who could be involved in extremism in Syria and Iraq.
"There are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with IS and other extremist organisations," Hammond said.
He said jihadists could pose a "direct threat to our own national security" if they seek to return with the "tradecraft" they have learnt in Iraq and Syria.
- Search for executioner's identity -
Shiraz Maher, from the International Centre for the Study of Radicalisation at King's College London, said British extremists are among the "most vicious and vociferous fighters" in the IS ranks.
He said the ICSR, which tries to track foreign fighters in Syria, was confident that Foley's executioner was from Britain "due to the colloquialisms" used, as well as his accent.
"We have a database of several hundred fighters in Syria and myself and my colleagues are frantically looking through it to try to pin him down," he added.
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The Guardian newspaper, citing an unnamed former hostage, claimed the executioner could be the leader of a group of "British fighters" that has guarded foreign hostages in Syria.
The paper reported the man was called John and hailed from London.
The newspaper cited linguistics experts who said the executioner has a "British accent, from the south" and spoke "multicultural London English".
Afzal Ashraf, an expert on counter-terrorism at the Royal United Services Institute security think-tank, told AFP that expert analysis, voice recognition technology and people who know the killer could soon establish his identity.
"There are people who are very good at pinpointing accents from particular communities -- not just geographical locations but also from ethnic backgrounds. You can tell a lot through the way people use words and their voice," he said.
Militants use British killer as propaganda
Islamic militants are using a beheading video to send a chilling message — not just through the gruesome act, but also by the choice of messenger.
The black-clad fighter who appears to kill journalist James Foley speaks with an English accent, underscoring the insurgents' increasing use of Western militants to mobilise recruits, terrify opponents and project the image of a global force.
He is the latest in a string of international jihadis — Britons, Australians, Chechens, Chinese and Indonesians — to appear in propaganda for the Islamic State group.
"They like to suggest they have a presence around the world much stronger than it is," said Charlie Cooper, a researcher at the Quilliam Foundation, a British counter-extremism think tank.
"It does suggest that people all over the world are going off to fight in the tens of thousands."
Morning news break – August 21