LONDON » The unmasking of Islamic State militant "Jihadi John" as a Londoner who had repeatedly been questioned by security services sent shock waves through Britain Friday, with Prime Minister David Cameron stepping in to defend British spy craft.

Cameron tried to defuse criticism of Britain’s intelligence community, which had "Jihadi John" on its list of potential terror suspects for years but was unable to prevent him from traveling to Syria, where he has played a prominent role in grisly beheading videos.

Cameron did not mention "Jihadi John" or refer to his real identity: Mohammed Emwazi, a Kuwait-born computer science graduate raised and educated in Britain. But he said the country’s spies make "incredibly difficult judgments" daily about how to pursue threats to national security and have broken up plots that would have caused immense damage.

Emwazi had been known to the British intelligence services since at least 2009, initially in connection with investigations into terrorism in Somalia.

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.

"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," he told the BBC.

The case has some parallels to that of two al-Qaida-inspired extremists who murdered a British soldier in a London street in May 2013. A report by lawmakers concluded that delays and other failings by the agencies had contributed to that tragedy.

However, it is not clear what laws could have been used to prevent Emwazi from leaving Britain at the time, since he had not been charged with any terrorist-related offenses. It is not known if police or security services had any evidence he was planning to join extremists in Syria.

His identification as the front man in IS murder videos has raised questions about how a soccer-playing London youngster who liked smart clothes became one of the world’s most wanted men.

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.

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 the group, 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 to link up with terrorists there.

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.

It is clear that Emwazi was unnerved after his unwanted interrogation.

"He knew everything about me; where I lived, what I did, the people I hanged around with," he wrote in one of the emails that CAGE made public Thursday. "He even said that he would try to visit me. But I refused and told him that I did not want him to pay me a visit."

The following year Emwazi accused British agents of preventing him from going to Kuwait, where he had a job and planned to marry. He wrote in one email that his "’life’ is kind of on a ‘pause.’"

Like many British Muslims who have become radicalized in recent years, he seemed to feel that Muslims were increasingly under attack in many parts of the world and complained to CAGE of the plight of his fellow believers 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. Four months later, police told them Emwazi was in Syria, CAGE said.

He 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.

Foley’s parents in Arizona on Thursday expressed surprise that "Jihadi John" was an educated man who had real prospects in life.

"So he, in a sense, had a privileged upbringing, so to me that makes that even more sad that he’d want to use his gifts for such evil and such hatred. It’s very frightening to me," Diane Foley said.

"We need to forgive him for not having a clue what he was doing," she said.

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 than killed by enemy action.

"People of his kind believe that death in combat is an honor, something special," she said.

In the modest west London neighborhood where Emwazi’s family lived, citizens were shocked after his identify was revealed.

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. He said Emwazi was "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.

Associated Press writers Jovana Gec in Belgrade, Serbia and Astrid Galvan in Tucson, Arizona contributed to this report.