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.
(Clockwise from left) Islamic State victims James Foley, Abdul-Rahman (Peter) Kassig, Alan Henning, Kenji Goto and Steven Sotloff 'The Beatles'
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".
- 1988: Born in Kuwait, moves to UK in 1994
- 2009: Completes computing degree at University of Westminster
- Aug 2009: Travels to Tanzania with two friends for safari but refused entry at Dar es Salaam. Put on flight to Amsterdam. After questioning there, returns to Dover
- Sept 2009: Travels to Kuwait to stay with father's family
- July 2010: Returns to UK for short stay but told he cannot return to Kuwait as visa denied
- 2012: Passes Selta English language teaching course
- 2013: Changes name by deed poll. Tries to travel to Kuwait but is stopped. Disappears. Parents report him missing. Police tell family four months later he has entered Syria
Mr Qureshi said Emwazi, who is understood to be about 27, had been "extremely kind, gentle and soft-spoken, the most humble young person I knew".
He said he could "not be 100% certain" Jihadi John was Emwazi although there were "striking similarities".
Journalists gathered outside a home in London where Mohammed Emwazi is believed to have once lived
Emwazi's difficulties began when he travelled to Tanzania in May 2009 following his graduation in computer programming at the University of Westminster, Mr Qureshi added.
He and two friends had planned to go on a safari but once they landed in Dar es Salaam they were detained by police and held overnight.
Emwazi then ended up flying to Amsterdam, in the Netherlands, where he claimed to be met by British intelligence agents from MI5 who accused him of trying to travel to Somalia, where the jihadist group al-Shabab operates. He denied the accusation and said the agents had tried to recruit him before allowing him to return to the UK.
A UK court document said Mohammed Emwazi was part of an extremist network linked to al-Shabab
We don't know exactly when the British or the American security services worked out that the masked man in the killing videos was Londoner Mohammed Emwazi - and nobody in official security circles is going to comment on how they got to that conclusion.
But we do know that he was, to use the jargon, a "person of interest" to MI5 going back to at least 2010 because he features in semi-secret court cases relating to extremism overseas and back in the UK.
Emwazi has been previously described as a member of a network involving at least 13 men from London - and at least two of them were subjected to house arrest control orders or T-Pims. One absconded. Another was killed in a drone strike. The chances of Emwazi ever returning to the UK are vanishingly small.
Emwazi later moved to Kuwait, where he got a job at a computer company. But on a visit to London in 2010, he was detained by British counter-terrorism officials and prevented from flying back to Kuwait, his friends said.
"I had a job waiting for me and marriage to get started," Emwazi wrote in a June 2010 email to Cage.
Asim Qureshi of the human rights group Cage describes Mohammed Emwazi as a "beautiful young man"
Mr Qureshi said Emwazi had made persistent efforts to try to change his situation: "We had two-and-a-half years of communications talking about what he could do to alleviate his problems."
He said he did not know what had happened to Emwazi, adding: "When we treat people as if they are outsiders they will inevitably feel like outsiders - our entire national security strategy for the last 13 years has only increased alienation. A narrative of injustice has taken root."
Emwazi was believed to have travelled to Syria around 2013 and later
joined IS, which has declared the creation of a "caliphate" in the large
swathes of Syria and neighbouring Iraq it controls.
A spokeswoman for Prime Minister David Cameron would not confirm or deny the latest reports, adding that the police and security services were working hard to find those responsible for the murder of the British hostages.
The University of Westminster confirmed Emwazi had left six years ago, adding: "If these allegations are true, we are shocked and sickened."
The BBC's Rajini Vaidyanathan in Washington says the FBI went on record last September to confirm it knew who Jihadi John was. However, US officials said this month they would not name him as they believed this would be the best strategy for finding him and bringing him to justice.
A spokesman for the family of Steven Sotloff said: "We want to sit in a courtroom, watch him sentenced and see him sent to a super-max prison."
David Haines's sister, Bethany, welcomed the identification, but told ITV News: "I think all the families will feel closure and relief once there's a bullet between his eyes."
Jihadi John sightings
A spokeswoman for Prime Minister David Cameron would not confirm or deny the latest reports, adding that the police and security services were working hard to find those responsible for the murder of the British hostages.
The University of Westminster confirmed Emwazi had left six years ago, adding: "If these allegations are true, we are shocked and sickened."
The BBC's Rajini Vaidyanathan in Washington says the FBI went on record last September to confirm it knew who Jihadi John was. However, US officials said this month they would not name him as they believed this would be the best strategy for finding him and bringing him to justice.
A spokesman for the family of Steven Sotloff said: "We want to sit in a courtroom, watch him sentenced and see him sent to a super-max prison."
David Haines's sister, Bethany, welcomed the identification, but told ITV News: "I think all the families will feel closure and relief once there's a bullet between his eyes."
- August 2014: Video in which US journalist James Foley is apparently beheaded
- 2 September 2014: Video in which US journalist Steve Sotloff is apparently beheaded
- 13 September 2014: Video in which British aid worker David Haines is apparently beheaded
- October 2014: Video in which British aid worker Alan Henning is apparently beheaded
- November 2014: Video in which Jihadi John is shown killing a Syrian soldier in a mass beheading, which also shows body of US aid worker Abdul-Rahman Kassig, also known as Peter Kassig
- 20 January 2015: Video in which Jihadi John is seen standing alongside two Japanese hostages and demanding a ransom in exchange for their release
- 31 January 2015: Video released appearing to show Jihadi John beheading Japanese hostage Kenji Goto