British Prime Minister David Cameron vowed to use all means at his disposal to hunt down militants such as Jihadi John after the killer was identified as a Kuwaiti-born computer programming graduate from London.
The black-clad militant brandishing a knife and speaking with an English accent was shown in videos released by Islamic State (IS) apparently decapitating hostages including Americans, Britons and Syrians.
When there are people anywhere in the world who commit appalling and heinous crimes against British citizens, we will do everything we can with the police, with the security services, with all that we have at our disposal to find these people and put them out of action, Cameron said.
Cameron refused to comment on the identification of Jihadi John as 26-year-old British militant Mohammed Emwazi, but said that people should get behind the security services, which he praised as impressive and dedicated to defending Britain.
Emwazi was known to the security services, which had tried to recruit him, according to prisoners' group Cage. The case has sparked debate about whether the security services let him slip through their grasp to join IS in Syria.
Dressed entirely in black, a balaclava covering all but his eyes and the bridge of his nose and a holster under his left arm, Jihadi John became a menacing symbol of Islamic State brutality and one of the world's most wanted men.
He used videos to threaten the West, admonish its Arab allies and taunt President Barack Obama and Cameron before petrified hostages cowering in orange jump suits.
Emwazi's name was first disclosed by the Washington Post. Two U.S. government sources who spoke on condition of anonymity said investigators believed Jihadi John was Emwazi.
The Sun and The Daily Mail newspapers published a picture showing a schoolboy Emwazi smiling and sitting cross-legged on the grass at the front of the photograph from the St Mary Magdalene Church of England primary school in Maida Vale, West London.
A picture of him as a student was later broadcast by Sky News, showing a young man with a moustache and goatee beard, wearing a cap with a logo resembling the P from the Pittsburgh Pirates US Major League Baseball team.
Born in Kuwait, Emwazi came to Britain aged 6 and graduated with a computer programming degree from the University of Westminster before coming to the attention of Britain's main domestic intelligence service, MI5, according to an account given by Asim Qureshi, research director of Cage, a group that campaigns for those detained on terrorism charges.
Emwazi, a fluent Arabic speaker, said MI5 had tried to recruit him and then prevented him from travelling abroad, forcing him to leave the country without telling his family, Qureshi told a news conference in London.
In a meeting with reporters, Qureshi cast Emwazi as a kind and thoughtful young man who faced harassment from MI5, which apparently suspected he wanted to join the Somali Islamist militant group al Shabaab.
That account prompted criticism of MI5. But there was little patience with the narrative of Emwazi's life presented by Qureshi, who called him a beautiful man.
Cameron's spokeswoman said it was completely reprehensible to seek to shift the blame from a killer to those seeking to keep British citizens safe.