A man with a British accent seen in ISIS videos showing the beheadings of Western hostages was identified Thursday as Mohammed Emwazi, a Kuwaiti-born Londoner, according to two U.S. officials and two U.S. congressional sources briefed on the matter.

The Washington Post first reported Emwazi as “Jihadi John,” citing one of Emwazi’s close friends.

A Muslim-led human rights advocacy group in London, CAGE, which had contact with Emwazi over alleged “harassment” by UK security services, also said in a statement there were “some striking similarities” between 26-year-old Emwazi and the man seen in execution videos holding a knife.

London’s Metropolitan Police declined to confirm the reported identity. CNN has been unable to confirm the reporting.

“We have previously asked media outlets not to speculate about the details of our investigation on the basis that life is at risk,” said Commander Richard Walton of the Metropolitan Police’s Counter Terrorism Command.

“We are not going to confirm the identity of anyone at this stage or give an update on the progress of this live counter-terrorism investigation.”

A UK Foreign Office spokeswoman told CNN: “We will neither confirm nor deny the current reporting as to the identity of Jihadi John.”

Reports indicate a middle class upbringing

The reporting in the Washington Post, quoted by Reuters, suggests Emwazi was from a middle-class family and grew up in London. He reportedly graduated from the University of Westminster in London with a degree in computer programming.

The Washington Post report, cited by Reuters, quoted unnamed friends of Emwazi as saying they believed his path to radicalization began when he went on a trip to Tanzania, in East Africa, in 2009 after graduating.

He was supposed to be going on safari there, but was reportedly detained on arrival, held overnight and then deported.

He was also detained by counterterrorism officials in Britain in 2010, Reuters said, citing the Post.

Emwazi is believed to have traveled to Syria in 2012, according to the Washington Post reporting cited by Reuters, and later to have joined ISIS there.

The advocacy group CAGE, in a statement on its website, said its research director Asim Qureshi had met with a Washington Post reporter who asked about the name Mohammad Emwazi.

CAGE’s files revealed that Qureshi had worked on Emwazi’s case “due to security service harassment,” the statement said.

“The following day, the journalist revealed to Qureshi that she knew from her own sources, that the man known as Jihadi John was Mohammad Emwazi. The journalist showed Qureshi a video of Jihadi John in order to identify him. Qureshi clarified that while there were some striking similarities, that due to the hood, there was no way he could be 100% certain,” the statement said.

Advocacy group blames UK security services

Emwazi was in communication with CAGE over the course of two years after being prevented from flying to Kuwait in 2010, the organization said.

Emwazi reportedly told CAGE at the time: “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.”

CAGE points the finger at British security services, who they say have “systematically engaged in the harassment of young Muslims, rendering their lives impossible and leaving them with no legal avenue to redress their situation.”

Qureshi drew a parallel with the case of Michael Adebolajo, convicted for the May 2013 murder in London of British soldier Lee Rigby, who said the UK domestic intelligence service MI5 had tried to recruit him.

Questions have been raised previously over whether CAGE, one of whose leading lights is Moazzam Begg, an ex-Guantanamo detainee, lends support to Islamist extremists.

On its website, CAGE says it is dedicated to securing the release of Aafia Siddiqui, nicknamed “Lady Al-Qaeda, who is jailed in the United States.

Haras Rafiq, managing director of the Quilliam Foundation, a UK-based counter-extremism think tank, told CNN that it was clear that Emwazi had been radicalized before 2010.

He said the intelligence agencies who stopped Emwazi traveling to Tanzania believed there was evidence that he intended to join the extremist group Al-Shabaab in Somalia.

Haras added that it was “very upsetting that an organization like CAGE would spin this in the way that they’ve done,” by blaming intelligence agencies.

‘Notorious celebrity’

Sajjan Gohel, director of international security at the Asia Pacific Foundation, told CNN that it had been an open secret for some time that the U.S. and British intelligence communities knew Jihadi John’s identity.

By not revealing it for operational reasons, he said, they may have created another issue.

“The problem was that it created more speculation in the media. In some ways, the nom de guerre of Jihadi John gave this individual a form of notorious celebrity,” he said.

The man’s reported background gives some clues, Gohel said. “We know that ISIS recruits a lot of Westerners who are skilled in new media, understanding of the Internet, because they use that as their platform as an oxygen of publicity.”

A University of Westminster spokeswoman said: “A Mohammed Emwazi left the University six years ago. If these allegations are true, we are shocked and sickened by the news. Our thoughts are with the victims and their families.”

Student safety is the university’s chief concern, it said, and it is, like other colleges in London, working to implement the UK government’s “Prevent” strategy to tackle extremism.

Familiar figure in videos

The masked, black-clad figure believed to be Jihadi John has become a familiar sight in ISIS’ gruesome beheading videos.

He appears to be the ISIS militant shown in a video last month demanding a $200 million ransom to spare the lives of two Japanese journalists. That man looks and sounds similar to one who has appeared in at least five previous hostage videos.

U.S. and British officials have previously said they believe they know who Jihadi John is, but they haven’t disclosed the information publicly.

That could be because Western intelligence agencies believe they have more to gain from keeping quiet, Aki Peritz, a former CIA officer, told CNN last month.

“They can put pressure on his family, put pressure on his friends,” he told CNN. “Maybe they have a line to him. Maybe they know who his cousins are who are going to Syria who can identify him. However, if you publicly tell everybody who he is, his real identity, then maybe he’ll go to ground and he’ll disappear.”