LONDON — Prime Minister David Cameron said Friday that Britain's intelligence services do a good job and make "incredibly difficult judgments," after revelations that the masked Islamic State militant known as "Jihadi John" had been on their radar for years.
Officials have identified the man shown in chilling hostage-beheading videos as Mohammed Emwazi, a Kuwait-born computer science graduate raised and educated in Britain.
Emwazi, now in his mid-20s, had been known to the British intelligence services since at least 2009, initially in connection with investigations into terrorism in Somalia.
His identification as the front man for IS murder videos has put pressure on British spies, who failed to stop him traveling to Syria, and raised questions about how a soccer-playing London youngster who liked smart clothes became one of the world's most wanted men.
David Anderson, who is in charge of reviewing Britain's terrorism legislation, said intelligence agencies may have dropped the ball, but faced a big challenge to identify real threats from "hundreds, probably thousands" of suspects.
He told the BBC that "perhaps they did slip up in this case but one won't know until there's been an inquiry or a report of some kind."
Cameron said the security services did an "incredibly impressive" job and in the last few months had stopped plots that could have caused "immense damage."
"All of the time, they are having to make incredibly difficult judgments and I think basically they make very good judgments on our behalf," he said.
Cameron did not mention Emwazi by name but said it was his top priority "to find these people and put them out of action."
Emwazi appeared in a video released in August showing the slaying of American journalist James Foley, denouncing the West before the killing. A man with similar stature and voice was also featured in videos of the IS killings of American journalist Steven Sotloff, Britons David Haines and Alan Hemming, and U.S. aid worker Abdul-Rahman Kassig.
Authorities were working to piece together the path to radicalization of Emwazi, who came to Britain from Kuwait as a small child and attended state schools in London before studying computer science at the University of Westminster.
British spies took an interest in him long before he traveled to Syria. Court documents from 2011 obtained by the BBC list Emwazi as part of a network of west London men suspected by MI5 of sending funds, equipment and recruits to al-Shabab militants in Somalia. The group included Bilail al-Berjawi, a Lebanese-British militant who was killed in a U.S. drone strike in Somalia in January 2012.
Emails that Emwazi sent to a Muslim advocacy group reveal a young man increasingly frustrated by the attentions of British spies and angry at the plight of Muslims around the world.
Emwazi approached CAGE after he and two friends were arrested and deported on a trip to Tanzania in August 2009. They said they were going on a post-university safari. But Emwazi said he was grilled by a British intelligence officer who accused him of trying to travel to Somalia.
He said the agent, who identified himself as Nick, suggested Emwazi "work for us" before saying "life will be harder for you" if he did not cooperate.
The following year Emwazi accused British agents of preventing him from going to Kuwait, where he had a job and planned to marry.
The emails show his increasing frustration with what he saw as official harassment. He wrote in one that "my 'life' is kind of on a 'pause.'"
He also sent CAGE messages complaining of the plight of Muslims in Chechnya, Iraq and elsewhere.
CAGE said that Emwazi even changed his name in a bid to escape the attentions of the security services, but still was barred from going to Kuwait. His family reported him missing early in 2013. CAGE said that four months later, police told the family Emwazi was in Syria.
CAGE is a controversial group that has been accused of condoning jihadi violence. On Thursday its research director, Asim Qureshi, described the Emwazi he knew as "extremely kind, extremely gentle."
The unmasking of "Jihadi John" sent ripples of shock through the modest west London neighborhood were Emwazi's family lived.
Sharaft Ullah, who worships at the Harrow Road Mosque near the family home, remembered Emwazi as a strict Muslim who prayed several times a day, "a very good local guy and polite with everybody."
"I feel angry because he was educated in this country and he graduated from Westminster," Ullah said. "If he has been doing these things it's wrong."
Another mosque that Emwazi was reported to have attended, the Greenwich Islamic Centre, said it had no knowledge of him.
The widow of Haines, a British aid worker, said Friday she would like to see "Jihadi John" captured and put on trial.
Dragana Haines told The Associated Press in a phone interview from her home in Croatia that "I really hope he will be caught, I think it would be a good lesson for all."
Haines, whose husband was killed in September, said she would rather see Emwazi judged in a court of law, because "people of his kind believe that death in combat is an honor, something special."
Associated Press writer Jovana Gec in Belgrade, Serbia contributed to this report.