They chose not to disclose his name earlier for operational reasons.
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 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.
Courtesy : BBC