The international intelligence community was working last night to identify a British jihadist murderer who beheaded US journalist James Foley.
Last night, Mr Foley's parents said they were "so proud" of their "fearless, courageous" son.
Speaking outside the family's home in New Hampshire, his father John said: "Jimmy's free and we know he's in God's hands and we know he's in Heaven, so we're so proud of him."
His mother Diane said Mr Foley "was a courageous, fearless journalist... he always hoped that he would come home".
She said her son, the oldest of five children, "had nine lives" and that even after more than 600 days since his kidnapping, the family "never gave up hope... we just ran out of time. He ran out of time".
Tearful, but self-possessed, Mrs Foley urged the media to put their son's story in a positive light, saying he "would never want us to hate or be bitter".
Asked how she and her family managed to deal with the chilling manner of their son's death, Mrs Foley responded: "Every time we started to get despondent, we thought of Jim and his courage."
President Barack Obama condemned the killing of the kidnapped reporter as "hateful", while David Cameron branded it "brutal and barbaric".
Meanwhile, sources on both sides of the Atlantic said they were deeply concerned about the fate of another American journalist, Steven Sotloff, who appeared at the end of the gruesomely explicit video showing Mr Foley's murder.
In the film the masked jihadist, who has an apparent London accent, warns that Mr Sotloff's life depends on Mr Obama's next move.
Mr Cameron, who, Downing Street confirmed, had watched the video, has instructed MI5 and MI6 to work with American intelligence agencies to try to identify the Briton in the video.
The suspect is believed to be among one of between 400 and 500 UK citizens fighting with Isis militants.
Last night it was claimed that he was the leader of a group of British fighters holding foreign hostages in Syria.
Sources in Syria said they recognised the man as one of a three-man team based in the Isis stronghold of Raqqa.
A former hostage, who was held for a year in Raqqa, described the British executioner as intelligent, educated and a devout believer in radical Islamic teachings.
He was said to be among three UK-born militants guarding foreign hostages. The group was referred to as 'The Beatles' by fellow hostages because of their nationality.
Mr Cameron, who returned to London from holiday yesterday, chaired a meeting of security chiefs to discuss the Government's response.
Afterwards he said it seemed "increasingly likely" that a British citizen was the killer.
However, Government sources added that it was still unclear whether the man heard speaking on the video was the same individual who carried out the murder because his face was obscured on the video.
Mr Cameron said: "Let's be clear what this act is – it is an act of murder, and murder without any justification.
"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."
The film shows Mr Foley (40), who went missing in Syria in 2012, kneeling in a desert-like environment at an unknown location as a 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 militant fighter stood next to him brandishing a knife.
Mr Foley had previously been held captive in Libya in 2011, but released after six weeks.
That experience, however, had not deterred him from covering the civil war in Syria.
James Foley was on an assignment for Agence France-Press and the Boston-based media company GlobalPost in northern Syria when his car was stopped by militants in 2012.
The 40-year-old freelancer from Rochester, New Hampshire, has not been heard from since. He is one of an estimated 20 journalists missing in Syria, according to the New York-based Committee to Protect Journalists.