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By Michael Holden and Mark Hosenball 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 who was known to the security services. 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. Two U.S. government sources who spoke on condition of anonymity confirmed to Reuters that investigators believed Jihadi John was Emwazi.