The masked militant first appeared in numerous gruesome videos put out by Islamic State, as Lucy Manning reports
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.
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.
(Clockwise from left) Islamic State victims James Foley, Abdul-Rahman (Peter) Kassig, Alan Henning, Kenji Goto and Steven Sotloff
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 four British jihadists guarding Westerners abducted by the group in Syria. They were given the nicknames "John", "Paul", "George" and "Ringo" by their captives, and were known collectively as "the Beatles".
Analysis: Dominic Casciani, BBC News
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.
But we do know that he was 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.
A UK court document said Mohammed Emwazi was part of an extremist network linked to al-Shabab
Nobody in official security circles is going to comment on what they know and why they know it.
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. The chances of Emwazi ever returning to the UK are vanishingly small.
BBC News special correspondent Lucy Manning says Emwazi is understood to be about 27 years of age.
Friends told the Washington Post that he was raised in a middle class area of West London and studied computer programming at the University of Westminster. He would on occasion pray at a mosque in Greenwich, they said.
Emwazi's friends told the Washington Post they believed he started to be radicalised after travelling to Tanzania in May 2009 following his graduation.
Journalists gathered outside a home in London where Mohammed Emwazi is believed to have once lived
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, they said.
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.
The incident is apparently described in a report published the following year by the Independent. It identified Emwazi as Muhammad ibn Muazzam.
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.
It is believed he was known to security services in the UK and the US before leaving for Syria and was linked to a man with connections to al-Shabab, says the BBC's Dominic Casciani.
"I had a job waiting for me and marriage to get started," Emwazi wrote in a June 2010 email quoted by the Washington Post.
"[But now] I feel like a prisoner, only not in a cage, in London," he added. "A person imprisoned and controlled by security service men, stopping me from living my new life in my birthplace and country, Kuwait."
The Washington Post said Emwazi was believed to have travelled to Syria around 2012 and later joined Islamic State, which has declared the creation of a "caliphate" in the large swathes of Syria and neighbouring Iraq it controls.
British police declined to comment on the reports, citing the "live counter-terrorism investigation".
A spokeswoman for Prime Minister David Cameron would not confirm or deny reports that Emwazi was Jihadi John, adding that the police and security services were working hard to find those responsible for the murder of the British hostages.
A White House spokeswoman referred journalists to the UK authorities.
Jihadi John sightings