US airstrikes against Islamic State (IS) extremists in Iraq continued as intelligence agencies on both sides of the Atlantic worked to identify the British jihadist believed to have carried out the brutal murder of American journalist James Foley.
Expert analysts are sifting through the gruesome footage for clues amid suggestions that the Islamic State (IS) extremist is from London.
The jihadist, referred to in the video as John, is reported to be the ringleader of a trio of UK-born extremists nicknamed after members of The Beatles responsible for guarding Western hostages, with the two others dubbed Paul and Ringo according to the BBC.
In the video of the murder of Mr Foley, US president Barack Obama was warned that the life of American hostage Steven Sotloff hinged on his "next decision".
But the US continued to launch air attacks on IS forces to shore up Kurdish and Iraqi efforts to hold the strategically important Mosul Dam.
British spies are heavily involved in efforts to identify Mr Foley's killer and his group of extremists, with the Cheltenham-based eavesdropping agency GCHQ playing a vital role in intercepting any electronic communications between jihadists in Syria and Iraq.
The British agencies - MI5, MI6 and GCHQ - were also working with American and European counterparts and receiving reports from agents on the ground to try and build a more complete picture of the scenario in Iraq and Syria.
However, much of the ongoing work will not deviate from everyday business as the hundreds of Britons who have travelled overseas to take up jihad have for some time been of central concern to the security services.
David Cameron, who returned to Cornwall on holiday after interrupting his break to take charge of the British response to the release of the murder video, said "far too many" Britons had travelled to the region to take part in jihad.
Foreign Secretary Philip Hammond said "significant numbers" of British nationals had been "involved in terrible crimes, probably in the commission of atrocities" with IS and other extremist groups.
Downing Street said the Prime Minister would continue to receive regular updates and briefings on the situation while he was in Cornwall.
A No 10 spokesman said: "The Prime Minister has returned to Cornwall after holding meetings in Downing Street yesterday on Iraq and Syria, following the murder of James Foley. He remains in close contact with his team and will be kept fully up to date."
Dr Andrew Mumford, a lecturer in politics and international relations at Nottingham University, said intelligence services faced a "big task" in trying to identify the murderer, and would be combing through people known to have links with Britons who have travelled to fight in Syria.
But he warned the agencies could face problems if the killer had travelled to the region "under the radar" with "no background in terrorism or any other criminal activities".
On the role of "The Beatles" in guarding kidnapped Westerners, he said: " A picture is emerging that IS is now explicitly using Western jihadists to take control and look after Western hostages. That is an important and interesting development."
A daring mission by US special forces this summer was launched in an attempt to rescue hostages including Mr Foley, senior officials in the Obama administration revealed.
The mission was authorised after intelligence agencies believed they had identified the location where the hostages were being held.
But several dozen special operations troops who were dropped by aircraft into Syria did not find them and engaged in a firefight with IS militants before departing.
The officials said a number of militants, but no Americans, were killed. One American suffered a minor injury when an aircraft was hit.
Both Mr Cameron and Mr Obama have condemned the murder of Mr Foley as "hateful" and "barbaric", insisting it would not force them to back away from tackling IS in Iraq and Syria.
After rushing back to Downing Street yesterday, Mr Cameron: "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. But we know that far too many British citizens have travelled to Iraq and travelled to Syria to take part in extremism and violence. And what we must do is redouble all our efforts to stop people from going.
"To take away the passports of those contemplating travel, arrest and prosecute those who take part in this extremism and violence. To take extremist material off the internet and do everything we can to keep our people safe. And that is what this Government will do."
Mr Obama said "no just God" could condone the killing of Mr Foley, who was seized in Syria in 2012, and IS would "fail" because they only wanted to destroy.
Mr Foley's family also paid tribute to his bravery.
Speaking alongside wife Diane at their home in Rochester, New Hampshire, John Foley said: "It's difficult to find solace at this point in time, but Jim is free. We know he's in God's hands and ... we know he's in heaven."
The murdered journalist's younger brother Michael criticised the American government, saying he thought it could have done more to save him.
One of the agencies Mr Foley worked for, GlobalPost, has said IS threatened to execute him a week ago and the US authorities were informed.
The film shows Mr Foley, 40, who worked for organisations including Agence France-Presse and went missing in Syria in 2012, kneeling in a desert-like environment at an unknown location as an IS fighter stands by his side dressed in black and with his face covered.
Pausing and taking deep breaths, the distraught journalist said he wanted to call on family, friends and loved ones to "rise up against my real killers, the US government" as the IS fighter stood next to him brandishing a knife.
Speaking with an apparently English accent, the killer accused America of "aggression towards the Islamic state", adding: "You have plotted against us and gone far out of your way to find reasons to interfere in our affairs."
He said further actions in Iraq by America "will result in the bloodshed of your people".
Mr Sotloff, who went missing near the border of Syria and Turkey last year, is also seen.
The footage has been authenticated by the US and UK, but Scotland Yard has urged people to avoid spreading it through Twitter and Facebook - warning that to do so could be a criminal offence.
The US is thought to be considering an additional deployment of troops to Iraq to help repel the IS offensive that has secured swathes of the country.
But Mr Cameron, who is understood to have watched the video himself, stressed there would be no "knee jerk" escalation of British military involvement - warning that the West faced a "generational struggle" against Islamist extremism.