The Islamic State militant known as "Jihadi John" has been identified as a Londoner called Mohammed Emwazi.
Emwazi was born in Kuwait and comes from a well-to-do family, according to the Washington Post.
He grew up in northwest London and graduated from the University of Westminster with a degree in computer programming.
Aged 27, he is believed to have travelled to Syria in 2012 and to have joined Islamic State (IS) later.
A close friend of Emwazi told the newspaper: "I have no doubt that Mohammed is Jihadi John. He was like a brother to me... I am sure it is him."
A representative of the British advocacy group CAGE who had been in contact with Emwazi before he left for Syria also said he believed he was "Jihadi John".
"There was an extremely strong resemblance," research director Asim Qureshi said after watching a video of him beheading a hostage.
He later described Emwazi as "extremely kind, extremely gentle... and the most humble young person that I ever knew".
Emwazi rose to notoriety last August in an IS video in which he killed American journalist James Foley.
Since then he has appeared in a series of videos documenting the gruesome murders of other hostages, including British aid workers Alan Henning and David Haines.
In each, he taunts audiences and is dressed all in black, with a balaclava covering his face. He wears a holster under his left arm.
The Washington Post claims Emwazi started to radicalise after a planned safari in Tanzania following his graduation was brought to an abrupt end. He was detained on arrival in Dar es Salaam and deported.
He is said to have told friends he was flown to Amsterdam where an officer from MI5 accused him of trying to reach Somalia, where the militant group al Shabab operates.