Mohammed Emwazi, a Kuwaiti-born British man believed to be from West London, who was known to UK security services.
They chose not to disclose his name earlier for operational reasons.
Mohammed Emwazi first appeared in a video last August, when he apparently killed the American journalist James Foley.
He was later thought to have been pictured in the videos of the beheadings of American journalist Steven Sotloff, British aid worker David Haines, British taxi driver Alan Henning, and American aid worker Abdul-Rahman Kassig, also known as Peter Kassig.
In each of the videos, the militant appeared dressed in a black robe with a black balaclava covering all but his eyes and top of his nose.
Speaking with a British accent, the man taunted and threatened Western powers before appearing to kill the hostages.
Last month, the militant appeared in a video with the Japanese hostages Haruna Yukawa and Kenji Goto, shortly before they were killed.
Friends of Mohammed Emzawi told the Washington Post that he was from a well-to-do family and that he studied computer programming at university.
Mohammed Emzawi is believed to be an associate of a former UK control order suspect, who travelled to Somalia in 2006 and is allegedly linked to a facilitation and funding network for Somali militant group al-Shabab.
The Washington Post said Mohammed Emzawi was believed to have travelled to Syria around 2012 and later joined ISIS, which has declared the creation of a “caliphate” in the large swathes of Syria and neighboring Iraq it controls.
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