The masked Islamic State (IS) militant nicknamed "Jihadi John" who appeared in beheading videos of Western hostages has been named as Mohammed Emwazi, a Kuwaiti-born British citizen in his mid-20s from West London.

Reuters reported that US government sources have confirmed that Emwazi, whose identity was first reported by the BBC and the Washington Post this morning, is the man who first appeared in an August 2014 IS video seemingly showing him murdering US journalist James Foley. He was later seen in similar footage showing the beheadings of two Americans — journalist Steven Sotloff, US-aid worker Abdul-Rahman (also known as Peter) Kassig — and two Britons — aid worker David Haines and taxi driver Alan Henning.

In each video, he is clad all in black with his identity hidden by a balaclava and taunts world leaders in a distinctive British accent. He was given the name by foreign hostages he once held as part of a team guarding Western captives.

His identity has been the result of intense speculation. UK intelligence services previously said they were aware of who he was, but didn't name him for operational reasons, while FBI director James B. Comey said in September last year, that after interviews with his captives and voice analysis, they believed they did also.

The family of Sotloff told the BBC that they were relieved their son's killer had now been identified.

Family of murdered US hostage Stephen Sotloff tells BBC's Jon Sopel of their relief he's now been publicly identified. — Frank Gardner (@FrankRGardner)February 26, 2015

But the British authorities refused to confirm or deny his identity today, saying they would not comment on an ongoing security operation.

Downing Street 'not going to get into the details of an ongoing police and security investigation. — Max Foster (@MaxFosterCNN)February 26, 2015

Emwazi grew up in a well-off family in West London, has a degree in computer programming from the University of Westminster and is thought to have travelled to Syria in 2012 before joining (IS), according to the Washington Post.

A crowd of reporters gathered this morning outside what is believed to be his family home in North Kensington. The directory 192.com has since removed the address concerned. VICE News understands that his family has been relocated by police.

Reporters outside block of flats where jihadi john was said to live. Not confirmed. — Hussein Kesvani (@HKesvani)February 26, 2015

A 21-year-old British-Bangladeshi who has been living in the area for a year and a half and is studying English language and literature to Westminster University, from which Enwazi graduated six years ago, said that students had been circulating articles on him today. She told VICE News she was "shocked and ashamed that he is from the same uni as us, to think that we had someone like that in our actual uni is awful."

She said she thought there would be a lot of speculation over the university environment though she did not believe it had played any role. " Personally i have never been influenced by any of that stuff being at Westminster University," she continued, adding: "Every time it's a Muslim person, the story is blown up so much more than with anyone else."

Friends described Emwazi as a polite and devout young man who enjoyed fashionable clothes, the Post said. They added that he started to show signs of more extreme beliefs in 2009 after a planned safari in Tanzania with two friends, which resulted in them being detained in Dar es Salaam then deported and flown to Amsterdam. There, Emwazi claimed that a member of British intelligence agency MI5 accused him of trying to reach Somalia, where extremist militants al-Shabaab operate, then tried to recruit him.

The safari episode is referenced in a 2010 Independent article that identifies Emwazi as "Muhammad ibn Muazzam". In it, both he and a friend say they were threatened with violence by the Tanzanian security forces on MI5 orders.

A BBC report linked Emwazi with a UK citizen suspected to be involved with al-Shabaab. He was referred to a UK court document dating back to 2010 as a member of a "network of UK and East Africa-based Islamic extremists which is involved in the provision of fund[s] and equipment for terrorism related purposes and the facilitation of individuals' travel to Somalia to undertake terrorist-related activities."

After this, Emwazi is reported to have had more run-ins with counterterrorism officers, including an incident where he was detained while trying to move to Kuwait.

An article published by CAGE disclosed that the human rights charity has been in contact with Emazi since 2009, and helped identify him as Jihadi John to the Washington Post.

The charity claimed that Emwazi had been "radicalized by Britain." They added that they had evidence that "there are several young Britons whose lives were not only ruined by security agencies, but who became disenfranchized and turned to violence because of British counter-terrorism policies".

Emwazi, 26, moved to the UK from Kuwait at six, the charity said. Outlining Emwazi's account of the Tanzania episode, it said that after completing his degree in 2009, the then 21-year-old made plans to go to the country with two friends.

Emwazi and his friends were refused entry to the country, had their passports confiscated and were detained in a cell for 24 hours for "informal questioning," the charity said.

The friends were eventually allowed to leave and boarded a plane to London via Amsterdam. On the stopover they were asked to leave the flight by four armed men and questioned by Dutch and British intelligence staff, CAGE said. The intelligence officers accused the three of attempting to reach Somalia from Tanzania. Emwazi said he had planned to go on holiday, at which point the British officer attempted to court him to work for MI5.

Eventually they were allowed to leave for the UK, but were detained again in Dover, the human rights charity claimed. Then in 2010, Emwazi tried to move to Kuwait, but found his visa rejected — in his belief, due to the interference of British authorities.

At that point he began communicating with CAGE on a regular basis, the charity said. In one letter, he wrote:

"I never got onto the flight, what was the point, I said to myself; I'll just get rejected. I had a job waiting for me and marriage to get started. But now I feel like a prisoner, only not in a cage, in London. A person imprisoned and controlled by security service men, stopping me from living my new life in my birthplace and my country, Kuwait."

Asim Qureshi, research director at CAGE, said: "Like Michael Adebolajo [one of the killers of British soldier Lee Rigby in London], suffocating domestic policies aimed at turning a person into an informant but which prevent a person from fulfilling their basic life needs would have left a lasting impression on Emwazi. He desperately wanted to use the system to change his situation, but the system ultimately rejected him."

At a news conference later, Qureshi said that he had known Emwazi from 2009 until January 2012, and described him as "extremely kind", "extremely gentle" and "the most humble person that I knew", adding that he occasionally dropped by the CAGE office to share "posh baklava".

He appeared to shed tears as he described Emwazi as a "beautiful young man", and said: "the person I have met would never hurt a single person."

Describing the alleged MI5 attempt to recruit Emwazi as an informant as a "fishing expedition", he said he believed that Emwazi's treatment at the hands of intelligence agency officials "almost definitely" played a part in his radicalisation.

He also revealed that Emwazi's family do not believe that he is Jihadi John, saying they were in total shock at the report.

CAGE immediately drew accusations of sympathy with extremists, including from Maajid Nawaz, a Liberal Democrat parliamentary candidate and head of the Quilliam Foundation, a counter-extremism think tank.

Haitham al-Haddad, a Saudi Arabia-born London-based cleric who has spoken in favor of female genital mutilation and reportedly described homosexuality as a criminal "scourge," was due to speak at an event at the University of Westminster tonight on the invitation of its Islamic Society, but VICE News understands the event has now been cancelled. A spokesperson for the university said they were "shocked and sickened" by the revelations the IS executioner was a former student.

Radical London-based preacher Anjem Choudary claimed to VICE News that Emwazi's unmasking had been deliberately timed to "synchronize" with British government policy.

"He [Emwazi] was affected by British foreign policy, similar to the Michael Adebolajo attack; he has been forced to act because of the British government. I think the naming of his identity is timed to synchronize with governmental policy. It is another attack to demonise the Muslim community and I wouldn't be surprised if raids, and more incrementing measures were implemented upon British Muslims."