Little is known about the man who killed journalist James Foley, 40, whose brutal death was depicted in a video posted online by Islamic State (ISIS) militants on Tuesday. But British Prime Minister David Cameron told the BBC that it is likely the killer is a British citizen -- a development that he described as "deeply shocking."

The footage of the execution, which was verified as authentic by the White House on Wednesday, shows a man in black clothes and face mask, standing next to Foley, who was on his knees. When the man spoke, his accent sounded distinctly English.

"We have not identified the individual responsible, but from what we have seen it looks increasingly likely that it is a British citizen,” Cameron told BBC News. “This is deeply shocking. But we know that far too many British citizens have travelled to Iraq and travelled to Syria to take part in extremism and violence. And what we must do is redouble all all our efforts to stop people from going."

U.S. and British intelligence officials were working to unmask the killer Wednesday. U.K. Foreign Secretary Philip Hammond told the Guardian, “We're very concerned by the apparent fact that the murderer in question is British and we are urgently investigating.”

Experts believe the man may be from the southeast of England, but most likely London, the Guardian reports.

Claire Hardaker, a linguistics expert at Lancaster University, said, "We're definitely looking at a British accent, from the south and probably from London, Kent or Essex. He does something interesting when he says 'Muslims.' You typically get 'Muzlims' but he says something closer to 'Musslims'."

Investigators are trying to pinpoint the location of the execution, and are analyzing the video for any further clues that might identify the killer.