British intelligence identify man they believe killed James Foley

British intelligence officials have identified the man they believe killed American journalist James Foley in Syria, U.K. security sources told NBC News on Sunday. NBC News has not yet confirmed the name of the suspect, and sources have said that rampant media speculation about the identity of the killer may be off base.

Earlier on Sunday, the British ambassador to the U.S. said officials were "close" to identifying the man with an apparent British accent who beheaded Foley in a video released by ISIS militants earlier in the week.

Freelance journalist James Foley
NBC News
Freelance journalist James Foley

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"We're not in a position to say exactly who this is," Sir Peter Westmacott said on NBC's "Meet the Press," but "I think we are close." Westmacott said "sophisticated" voice recognition technology was being used to identify the man who appeared in the video, but he also stressed that the threat of British nationals in ISIS goes beyond one killer.

"It's not just about one brutal murderer," Westmacott said. British intelligence estimates that about 500 Britons have joined ISIS, meaning they have the means to return to Britain or fly to America with few hindrances. "It is a threat to our citizens," Westmacott said.

A U.S. Department of Justice official had told NBC News earlier Sunday that investigators were using "all of the tools at our disposal" to hold Foley's killers "accountable."