The masked executioner who as 'Jihadi John' has come to embody the brutal violence of Isis has been identified as a British computer programmer who was known to MI5 and police for at least four years before he slipped away into Syria.
The revelation that Mohammed Emwazi, a Kuwait-born Briton who lived in London for 17 years, was the figure appearing in the succession of grim online videos in which Western hostages are beheaded has raised fresh questions about the ability of the security services to monitor extremists.
Emwazi drifted between jobs as a computer programmer and made efforts to move abroad after gaining a foreign-language teaching qualification. But he seems to have succeeded in leaving the UK in 2013 for Syria after changing his name to Mohammed al-Ayan by deed poll.
The 26-year-old had first come to the attention of MI5 in 2009. Then he was detained with two others while travelling to Tanzania for a safari holiday, which intelligence officers believed was a cover for an attempt to join the al-Shabaab terror group in Somalia.
His week-long detention in Africa, apparently on British orders, and questioning by an MI5 officer were revealed in 2010 - although he then identified himself by a different name. Campaigners from the human rights group Cage claimed yesterday that Emwazi then became the subject of a campaign of harassment by MI5 to try to persuade him to become an informant between 2010 and 2012.
Asim Qureshi, a director with Cage, said he had come to know "beautiful young man" Emwazi before he fled for Syria.
Mr Qureshi said the country's national security policy "only increased alienation" since the 9/11 terrorist attacks, and cited similarities between the case of Emwazi and that of Michael Adebolajo, who murdered soldier Lee Rigby in Woolwich in 2013.
He spoke after Sir Menzies Campbell, a member of the Intelligence and Security Committee (ISC), said the parliamentary committee was likely to seek answers from the security services after the general election about what information they held on Jihadi John.