Prime Minister David Cameron has defended Britain's security services after Islamic State executioner "Jihadi John" was unmasked as London graduate Mohammed Emwazi, a man previously monitored by spy agency MI5.
As more details emerged about Emwazi, named by media and experts as the man who beheaded at least five Western hostages held by the IS group in Syria and Iraq, questions were asked about whether he could have been stopped.
Civil rights group Cage, which had been in touch with the Kuwaiti-born computing graduate before he left Britain, said MI5 had been tracking Emwazi since at least 2009.
Officials have not confirmed Emwazi's identity, but Cameron said Friday: "We will do everything we can with the police, the security services, with all that we have at our disposal, to find these people and put them out of action."
He added: "All of the time they (the security services) are having to make incredibly difficult judgements and I think basically they make very good judgements on our behalf."
Olivier Guitta, managing director of security and risk consultancy GlobalStrat, warned the security forces lacked the resources to track all those who crossed their radar.
"To monitor one person you need 30 officers, so if you have in England 1,000 people that are on your list, you need 30,000 officers. We don't have that," he told AFP.
But senior lawmaker David Davis, a member of Cameron's Conservative Party, said Emwazi was known to associate with fanatics and was on a terror watchlist.
"How many more people must die before we start to look more closely at the strategy of our intelligence services?" he wrote in The Guardian newspaper on Saturday.
Further criticism came from Cage, a group which supports people impacted by the so-called war on terror, which said Emwazi had been radicalised because of "harassment" by British intelligence agents.
The group said this began following a post-graduation trip to Tanzania in 2009 when Emwazi was accused of seeking to join militants in Somalia.
Cage also alleged that MI5 tried to recruit the graduate, who it described as a "beautiful young man".
Cameron's office said it was "reprehensible" to suggest MI5 was to blame for Emwazi's actions, while London mayor Boris Johnson accused Cage of an "apology for terror".
John Sawers, the former head of Britain's MI6 foreign intelligence agency, told the BBC: "The idea that somehow being spoken to by a member of MI5 is a radicalising act, I think this is very false and very transparent."
Emwazi had lived in Britain since the age of six and many of those who knew him expressed disbelief at his new identity.
One of his former high school teachers told the BBC that Emwazi had anger issues as a teenager but was successfully helped to control his emotions.
The unnamed woman said he "had everything going for him" and said his apparent transformation was "unbelievable".
She also said that MI5 had questioned some of his former teachers this week.
An acquaintance of Emwazi who worshipped at a mosque near his London home meanwhile described him as a "strict" Muslim who prayed up to five times a day, and a "good guy".
In the gruesome Islamic State videos posted online, "Jihadi John" is dressed all in black with only his eyes exposed, brandishing a knife while launching tirades against the West.
It is a stark contrast from a photograph from his student days, obtained by Sky News, that shows him with a goatee beard and wearing a Pittsburgh Pirates baseball cap.
Relatives of the slain IS hostages expressed hope that he will now be brought to justice.
"My only hope is that the revelation of his identity will lead to his arrest," Dragana, the widow of murdered British aid worker David Haines, told AFP from her home in Croatia.
However, Haines' daughter Bethany said she would feel closure only "once there's a bullet between his eyes".
"Jihadi John" is also believed to have killed US journalists James Foley and Steven Sotloff, British aid worker Alan Henning and US aid worker Abdul-Rahman Kassig.
He also appeared in a video with Japanese hostages Haruna Yukawa and Kenji Goto shortly before they were killed.