Intelligence services could soon know the identity of the killer of US journalist James Foley, with suggesting the south London-accented jihadi may be linked to an NHS doctor once accused of helping to kidnap Westerners in Syria.
And several former captives held by the British jihadists - dubbed 'The Beatles' - have come forward to say they can help identify him.
The jihadist in the execution video, which shows Foley kneeling in the desert with his head shaved and wearing an orange jumpsuit before he is beheaded, is believed to be the ringleader of a group of UK-born extremists, and using the name 'John'. They are apparently responsible for guarding Western hostages in the stronghold of Raqqa, and nicknamed after members of the Beatles, with the two others dubbed Paul and Ringo.
Footage issued by Islamic State militants that shows 'jihad John'
Didier François, a French journalist who was held by IS until April and spent time with Foley, told Europe 1 Radio he "roughly" recognised the killer, but feared any information he gave to the media or intelligence services would result in reprisals against captives.
"Recognised is a very big word. I see roughly who it is," he said, but added: "If you make public the fact they are being held or that you were together, reprisals will follow against them. Their exact words were: 'They’ll be punished.'"
Released French hostage Didier Francois, with French President Francois Hollande
The Telegraph reported that former St Bart's hospital doctor Shajul Islam, who was arrested and charged with kidnapping British journalist John Cantlie in 2012, could have links to the man now dubbed 'Jihad John'.
The case collapsed, with Cantlie unable to give evidence, and Islam has maintained he was on a humanitarian mission to Syria. Another British jihadist, also from Stratford and known to have travelled to Syria, Jubayer Chowdhury was also charged with kidnapping Westerners, but the case was also droppped.
Other names touted as potentially being connected to the killer are Islam's younger brother Rajul, who is believed to currently be in Syria, Aine Davis, a West London Muslim convert and former gang member, and Maida Vale rapper Abdel-Majed Abdel Bary who has tweeted pictures of himself holding a decapitated head.
Aine Davis, a Muslim convert and ex-drug dealer who is fighting in Syria
The probe into Foley's killer comes as the debate raged over whether tougher surveillance should be in place to track young men going to fight in Syria and Iraq. Lord Carlile, former independent reviewer of terrorism legislation for the Government, said the decision to scrap the control order system in favour of more limited terrorism prevention and investigation measures (Tpims) should be revisited.
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"I do think the Government could make a legislative response to the current problem by reintroducing control orders, or beefed-up Tpims, as they are called, to ensure that people who are identified by solid intelligence as presenting this kind of risk can be placed under controls which can prevent them activating their ideas," he said.
Lord Carlile said the Government needed to "spend more money" on campaigns to prevent radicalisation.
In some parts of the country, such as Cardiff, key schemes were "just vestigial", and had allowed developing extremists to "slip through the net".
Former Middle East minister Alistair Burt also called for the reinstatement of control orders. "If the authorities believe someone to be dangerous, what sort of monitoring is possible of that individual?" he told the Guardian.
"That raises the issue of control orders coming back on to the agenda again. It is time to revisit control orders. These were cancelled when we came into office. But circumstances have changed."
General Martin Dempsey, chairman of the US joint chiefs of staff, said IS was "an organisation that has an apocalyptic, end-of-days strategic vision and which will eventually have to be defeated".
"Can they be defeated without addressing that part of their organisation which resides in Syria? The answer is no. That will have to be addressed on both sides of what is essentially at this point a non-existent border."