LONDON/WASHINGTON (Reuters) - The masked "Jihadi John" killer who fronted Islamic State beheading videos has been identified as Mohammed Emwazi, a British computer programming graduate from a well-to-do London family.

The black-clad militant brandishing a knife and speaking with an English accent was shown in videos released by Islamic State (IS) apparently decapitating hostages including Americans, Britons and Syrians.

The 26-year-old militant used the videos to threaten the West, admonish its Arab allies and taunt President Barack Obama and British Prime Minister David Cameron before petrified hostages cowering in orange jump suits.

Emwazi's name was first disclosed by the Washington Post, citing unidentified former associates, but two U.S. government sources who spoke on condition of anonymity confirmed to Reuters that investigators believed Jihadi John was Emwazi.

Dressed entirely in black, a balaclava covering all but his eyes and the bridge of his nose and a holster under his left arm, Jihadi John became a menacing symbol of Islamic State brutality and one of the world's most wanted men.

Hostages called him John as he and other Britons in Islamic State had been nicknamed the Beatles.

Emwazi was born in Kuwait but came to Britain aged 6 and graduated with a computer programming degree from the University of Westminster before coming to the attention of Britain's main domestic intelligence service, MI5.

An associate of Emwazi, who was a fluent Arabic speaker, said MI5 tried to recruit him and then prevented him from traveling abroad, forcing him to flee abroad without telling his family. He traveled to Syria around 2012.

MI5 does not publicly comment on such claims. The British government and police refused to confirm or deny his identity, citing an ongoing security investigation.

"We don't confirm or deny matters relating to intelligence," said a spokeswoman for Cameron, who has ordered spy agencies and soldiers to track down the killer.

MOST WANTED MAN

"Jihadi John" rose to notoriety in August 2014 when a video appeared showing a masked man raging against the United States before apparently beheading U.S. citizen James Foley off camera.

View gallery'Jihadi John' killer from Islamic State beheading videos …
A masked Islamic State militant holding a knife speaks next to man purported to be U.S. journalist J …
Intelligence services in the United States and Britain used a variety of investigative techniques including voice and facial recognition as well as interviews with former hostages to identify the man, with whom MI5 had already had dealings.

But security officials made great efforts to avoid publicly naming Emwazi, fearing that would make him more difficult to catch. They were uneasy that the name was revealed.

There was no answer at two addresses in west London where Emwazi was listed to have lived. Neighbors described the family as "normal people" and "friendly".

"This is the first time anything like this happens in this neighborhood," said Fatima Al-Baqali. "We have to be careful now. I didn't know this family and I usually know everyone here."

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