Investigators believe that the Jihadi John masked fighter who fronted Islamic State (IS) beheading videos is a British man named Mohammed Emwazi, according to two US government sources.
He was born in Kuwait and comes from a prosperous family in London, where he grew up and graduated with a computer programming degree, according to the Washington Post.
In videos released by IS, the black-clad militant brandishing a knife and speaking with an English accent appears to have decapitated hostages including Americans, Britons and Syrians.
The Washington Post said Emwazi, who used the videos to threaten the West and taunt leaders such as Barack Obama and British prime minister David Cameron, was believed to have travelled to Syria around 2012 and to have later joined IS.
In each beheading video, he is dressed entirely in black, a balaclava covering all but his eyes and the ridge of his nose. He wears a holster under his left arm.
Hostages gave him the name John as he and other IS Britons had been nicknamed the Beatles. Another was dubbed George.
British government sources and the police refused to confirm or deny the report, citing a live anti-terrorism investigation, a position mirrored by a spokeswoman for Mr Cameron.
"We don't confirm or deny matters relating to intelligence," the spokeswoman said. "I am not going to get into the details of an ongoing police and security investigation.
"We have said since we have seen the awful actions being taken by these terrorists that we are absolutely determined to bring the perpetrators to justice, and the police and the security agencies have been working hard to do that, continue to work hard to do that and that is what we want to see."
Since a video surfaced last August showing a masked man raging against the United States before apparently beheading US citizen James Foley off-camera, Jihadi John has been one of the world's most hunted men.
Intelligence services in Britain and the United States were ordered to track down the masked man who became a menacing symbol of the brutality of IS. Authorities used a variety of investigative techniques including voice and facial recognition as well as interviews with former hostages.
The services had chosen not to disclose his name for operational reasons.
There was no answer at addresses in London where Emwazi was listed as living.
The Post quoted one of Emwazi's close friends as saying: "I have no doubt that Mohammed is Jihadi John. He was like a brother to me. I am sure it is him."
The Post quoted the friends of Emwazi, who spoke on condition of anonymity, as saying they thought he had started to become radicalised after a planned safari in Tanzania following his graduation from the University of Westminster in London.
They said Emwazi and two friends - a German convert to Islam named Omar and another man, Abu Talib - never made it to the safari. On landing in Dar-es-Salaam in May 2009, they were detained by police and held overnight before eventually being deported, they added.
In a statement, the University of Westminster said a Mohammed Emwazi had left the college six years ago.
"If these allegations are true, we are shocked and sickened by the news," a spokesperson said.
The Post said Emwazi claimed that representatives from Britain's MI5 security service had tried to recruit him. He later tried to move to Kuwait but was detained by counter-terrorism officials in Britain in 2010.
Meanwhile, the group that helped name Mohammed Emwazi as IS executioner Jihadi John caused outrage yesterday after blaming MI5 for pushing him towards his killing spree in Syria.
Asim Qureshi, research director at the rights group CAGE, described the world's most wanted man as "extremely kind, extremely gentle and the most humble young person that I ever knew".
At a press conference yesterday, Mr Qureshi was close to tears as he said Emwazi, who is linked to a string of executions of IS prisoners, "was such a beautiful man".
He said Emwazi was radicalised after the security services harassed him and alienated him because "the man I knew would never hurt a single person".
"When we treat people as if they're outsiders, they are going to feel like outsiders and they will look for belonging elsewhere," he said.
The assertion that MI5 "alienated" Emwazi caused fury online, accusing CAGE of being apologists for "barbarism".