The Islamist 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 Mr. 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 Federal Bureau of Investigation.
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 Centre for the Study of Radicalization at King's College London, said it was likely Mr. Foley's beheading took place in Raqqa, a stronghold of the Islamic State militant group and the base for many of its foreign fighters.
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.
Mr. 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 meant they don't talk about football clubs," he said.
Mr. 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.
"The person is taking great care to do 'posh talk,' as it were," Mr. 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.
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.
British Muslim leaders have condemned Mr. 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," Mr. Barrett told the BBC.
Meanwhile, a U.S. official said the Islamic State militants who beheaded Mr. Foley had demanded $132.5-million – or 100 million euros – in ransom for his release.
A second U.S. official says the demands were sent in e-mails to Mr. Foley's family in New Hampshire. Both officials spoke Thursday on condition of anonymity because they were not authorized to discuss the ransom demands by name.
Separately, Mr. Foley's former employer said that the militants first demanded the money late last year.
GlobalPost CEO Philip Balboni also indicated that European governments paid far less to have the Islamic State release their hostages.
U.S. policy prohibits the government negotiating with terrorists, including paying ransom.