Prime Minister David Cameron defended Britain's secret services and vowed to defeat jihadists Friday after media reports named Islamic State executioner "Jihadi John" as London graduate Mohammed Emwazi.
"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," Cameron said at a press conference in Wales.
"Jihadi John", believed to be the IS militant responsible for beheading at least five Western hostages, was identified Thursday by media and experts as a Kuwaiti-born computing graduate who had lived in London since the age of six.
As families of the slain hostages called for justice, Britain's domestic spy agency MI5 came under scrutiny following revelations Emwazi had been known to security services for several years.
"I work very closely with our security services, I meet with them regularly, I ask them searching questions about what they do," Cameron said.
"While we are in the middle of this vast effort to make sure British citizens are safe, the most important thing is to get behind them," he added.
- 'MI5 blunders' -
Civil rights group Cage said MI5 had been tracking Emwazi, aged in his mid-20s, since at least 2009.
"MI5 blunders that allowed Jihadi John to slip the net," read a headline in the Daily Telegraph newspaper, while the Daily Mail asked: "On the MI5 watch list, so how could he escape to Syria?"
Olivier Guitta, managing director of security and risk consultancy GlobalStrat, warned that British security forces lacked adequate resources to track all those who crossed their radar.
"You can follow a guy for one year, two years, he doesn't do anything so you have to drop it," he told AFP.
"To monitor one person you need 30 officers, so if you have in England a thousand people that are on your list, you need 30,000 officers. We don't have that."
Menzies Campbell, a member of parliament's intelligence and security committee, told BBC radio that lawmakers would look into the issue, adding: "There's no doubt that from time to time the security services have got to prioritise those upon whom they are conducting surveillance".
- 'Apology for terror' -
A spokeswoman for Cameron's Downing Street office said it would not comment on "speculation" about the identity of "Jihadi John" and said the government's priority was "bringing the murderers to justice and ensuring we protect any British citizens at risk".
London mayor Boris Johnson meanwhile accused Cage of "an apology for terror" for blaming Emwazi's radicalisation on his alleged detention and "harassment" by the British security services.
Cage, which published years of correspondence with Emwazi, said he had become radicalised following a post-graduation trip to Tanzania in 2009 when he was accused of seeking to join militants in Somalia.
It also alleged that MI5 had launched a failed bid to recruit him.
"It was incredible that people could stand up and pretend that somehow it was the fault of the security forces," Johnson said.
Guitta expressed concern about the number of people being radicalised in the West, saying that in giving forums to extremist preachers, "we are allowing people to put poison in the mind of those people".
In the gruesome Islamic State videos posted online, the masked executioner appears dressed all in black with only his eyes exposed, brandishing a knife while launching tirades against the West.
"Jihadi John", nicknamed after Beatle John Lennon due to his British accent, is believed to be responsible for the murders of US journalists James Foley and Steven Sotloff, British aid workers David Haines and Alan Henning and American aid worker Abdul-Rahman Kassig.
He also appeared in a video with Japanese hostages Haruna Yukawa and Kenji Goto shortly before they were killed.
Families of the slain Western hostages said they hoped that the naming of the masked killer would help bring him to justice.
"My only hope is that the revelation of his identity will lead to his arrest and that he will be judged according to civilised law," Dragana, the widow of David Haines, told AFP from her home in Croatia.
But Haines' daughter Bethany told British news channel ITV that victims' families would feel closure only "once there's a bullet between his eyes".