As horror turned to anger at the callous murder of US journalist James Foley, intelligence services are compiling information on his killer, who is believed to be a British jihadi.
Early clues from the gruesome footage suggest that the Islamic State (IS) extremist - referred to in the video as 'John' - is from London and is left-handed.
It was revealed in the aftermath of Foley's beheading, which was filmed and distributed on social media, that US President Barack Obama sent troops to Syria this summer to rescue a number of Americans being held hostage, including Foley because they believed they had identified the location where the hostages were being held.
But the 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.
Information is scarce about the identity of the man the Sun newspaper dubbed the "Ali G jihadi". But here's what we do know:
He was part of a group of four Brits called 'The Beatles' based in the ISIS stronghold of Raqqa
A former hostage, who was held for a year in the Syrian town of Raqqa, has told the Guardian that the killer was the ringleader of a trio of UK-born extremists the captives nicknamed "The Beatles" because of their nationality.
He was the ringleader, and in charge of guarding foreign hostages
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The masked killer who murdered Foley is known as 'John' to the group.
He is left-handed
Only 10% of the world's population are left-handed. All of the information from the video will be analysed rigorously by intelligence services, including the way he holds his weapon, his height, body movement and intonation. MI5 have a database of Brits they believe have travelled to Syria, and they will be comparing what they know about each one, the Telegraph reported.
He is probably from south London but could have family links to Afghanistan
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Dr Claire Hardaker, a linguistics experts at Lancaster University, has told several media outlets that the man's vowels marked him out as likely from the south-east of England, but most likely from London. Elizabeth McClelland, a forensic voice and speech analyst, told the Telegraph the accent has "possible influences of Farsi, which could suggest a family link to Afghanistan".
He was probably chosen for the job because his British accent would be more sinister for Western viewers of the video
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"This is significant because it signifies a turn towards threatening the west. They are saying we're going to come after you if you bomb us," Prof Peter Neumann, director of the International Centre for the Study of Radicalisation, at King's College London told the Guardian.
He emailed the Foley family, furious about the US airstrikes, informing them he would kill their son
Foley's family had been emailed by ISIS as early as last Wednesday and were informed that the terror group intended to execute the reporter in retaliation for US air strikes against Isis targets in northern Iraq.
GlobalPost chief executive, Philip Balboni said that ISIS "made no demands", just informed the family the execution was going to take place. They tried to engage him in conversation, but to no avail, because the jihadist was fuelled by "seething anger".
He had previously wanted a ransom to spare Foley's life, but the US government did not pay
According to the New York Times who spoke to a family representative and a captive held alognside Foley, ISIS demanded the United States to provide a $100 million ransom ransom for Foley's life, but unlike several other European countries who did pay out, the US refused.
He was the main negotiator in the release of 11 IS hostages earlier this year
Almost a dozen hostages, some held for over six months, were handed to Turkish officials. They included two Spanish journalists, one pictured here, Javier Espinosa.
The militants foiled an attempted rescue by US Special Ops
US President Barack Obama sent troops to Syria this summer to rescue a number of Americans being held hostage, including Foley, senior administration officials said. 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 killer treated Foley differently and more harshly that other hostages
French journalist Nicolas Henin spent seven months in captivity with Foley, including a week where they were handcuffed together, telling the BBC Foley was treated as "some kind of scapegoat" and was beaten more frequently. "Some countries like America but also like the UK do not negotiate and, well, they put their people at risk," he said.