The Islamic militant in a video showing the death of American journalist James Foley took great care to disguise his identity, dressing head-to-toe in black, with a mask leaving only his eyes visible.
But police and intelligence services in Britain and the United States have a plethora of clues as they scramble to identify him, from image analysis and voice-recognition software to social media postings and testimony from former captives.
Prime Minister David Cameron has said the masked jihadi pictured holding a knife to Foley's throat is likely British, and linguists say his accent suggests he is from the London area.
Britain's Metropolitan Police is involved in the hunt for him, as are British intelligence agencies and the FBI.
The Guardian newspaper on Thursday quoted an unnamed former captive who was held by the Islamic State group in Raqqa, Syria, as saying he appeared to be one of several British militants — nicknamed "The Beatles" by hostages — charged with guarding prisoners.
Peter Neumann, director of the International Center for the Study of Radicalization at King's College London, said it was likely Foley's beheading had taken place in Raqqa, a stronghold of the Islamic State militant group and the base for many of its foreign fighters.
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He said investigators would use basic detective techniques to narrow down the field of suspects before turning to voice recognition software and other sophisticated technology.
Neumann said most of the hundreds of Western militants in Syria have Facebook or Twitter accounts, on which they post pictures of themselves and give away other clues to their origins, such as a favourite soccer team.
"Just because they are Islamic extremists and behead people doesn't mean they don't talk about football clubs," he said.
Let guard down
Neumann said online photos could be analyzed to determine height, weight, eye colour and other information.
He said that even though the militants, most in their teens and 20s, know they should be careful, they are so ingrained in online culture that "they let their guard down."
"These are all young guys," he said. "It's a habit. It's very hard for them to be disciplined."
Language experts say the masked killer sounds like a man in his 20s who was raised or educated in Britain. John O'Regan, a linguist at the University of London's Institute of Education, said the militant spoke with a "multicultural London English" accent but with more formal standard English pronunciation, suggesting that his words denouncing American actions in the Middle East had been carefully scripted.
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"The person is taking great care to do 'posh talk,' as it were," O'Regan said. "They're very mindful of their p's and q's."
He said that even though the speech differs from the man's normal speaking voice, "there are enough features in the accent" to provide strong clues to his identity.
Voices can change
British spy agencies have access to voice-recognition technology that can try to match features of the killer's voice to a pre-existing recording, such as an online video or intercepted phone call.
So-called "voiceprinting" isn't perfect — voices can change over time and the technology isn't as reliable as fingerprinting or DNA. But Western counter-terrorism officials have long made use of it — including in Britain, where surveillance aircraft have flown over cities in a bid to match recordings of terrorist suspects against live phone conversations.
Human intelligence may be at least as important as technology in tracking down the masked man.
British Muslim leaders have condemned Foley's slaying and urged anyone who knows the killer to speak to the authorities. Richard Barrett, a former head of counterterrorism for Britain's MI6 intelligence agency, said the militant would likely be recognized by "the intelligence community … but also the community from which this man comes."
"He will have had many acquaintances and friends in the United Kingdom and those people will wish to see him brought to justice," Barrett told the BBC.
Captors demanded $135M ransom
Foley's death, highlighting how Syria has become perhaps the most dangerous country on earth for journalists, followed intense efforts by GlobalPost and others to identify his captors, and happened despite brief email exchanges between the militants and his family in late 2013 about a possible ransom.
The captors demanded a ransom of 100 million euros, or about $135 million, for his release, according to GlobalPost, a Boston-based online news publication that employed Foley.
The White House declined to comment on the warning about Foley but it said special operations troops were sent to Syria earlier this summer on a secret mission to rescue American hostages, including Foley, but did not find them.
"Since his capture, we have been using every tool at our disposal to try to bring him home to his family and to gather any and all information we could get about his whereabouts, his condition and the threats he faced," White House spokeswoman Caitlin Hayden said.
Obama vowed on Wednesday the United States would keep supporting Iraqis in the fight against Islamic State.