The Islamic State militant shown beheading US journalist James Foley is believed to be from Britain and nicknamed 'John', according to various newspaper reports.
The man is thought to be a senior figure in the jihadist group and is a prime target for MI5, MI6 and the SAS, according to the Times.
The paper claims he is linked to a group of British-Pakistani hostage specialists jokingly known as the Beatles - prompting the nickname from a former captive.
The Telegraph reports that US and UK intelligence services are now urgently investigating the claims about the "well-educated" and "intelligent" killer - who is thought to be based in IS-stronghold Raqqa, Syria.
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.