The masked Islamic State militant known as "Jihadi John", who has been pictured in the videos of the beheadings of Western hostages,has been named.
He is Mohammed Emwazi, a Kuwaiti-born British man in his mid-20s from west London, who was previously known to British security services.
British police declined to comment, citing ongoing investigations.
Emwazi first appeared in a video last August, when he apparently killed the US journalist James Foley.
He was later thought to have been pictured in the videos of the beheadings of US journalist Steven Sotloff, British aid worker David Haines, British taxi driver Alan Henning, and American aid worker Abdul-Rahman Kassig, also known as Peter.
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, he taunted Western powers before holding his knife to the hostages' necks, appearing to start cutting before the film stopped. The victims' decapitated bodies were then shown.
Earlier this month, the militant featured in a video in which the Japanese journalist Kenji Goto appeared to be beheaded. Hostages released by IS said he was one of three British jihadists guarding Westerners abducted by the group in Syria. They were known collectively as "the Beatles".