Hunt for James Foley's Islamic State killer
21 August 2014 Last updated at 14:16 BST
The intelligence services and the police are trying to identify the masked man, believed to be British, filmed murdering the American journalist James Foley.
It has also emerged that foreign hostages held by Islamic State in Syria, are being guarded by three British jihadists, nicknamed: John, Paul and Ringo.
The White House has revealed US special forces launched a secret mission this summer to rescue American hostages in Syria, including Mr Foley, but could not find them.
The BBC's Home Affairs correspondent Dominic Casciani reports.