LONDON — The Islamic State executioner with the British accent has been unmasked as a Kuwaiti-born Londoner who grew up in affluence but turned to militancy and barbarism after repeated harassment by security agencies simply because, in his view, he was a Muslim.

The tall man who beheaded hostages in a series of Islamic State videos and earned the name "Jihadi John," was identified by The Washington Post and other news organizations as Mohammed Emwazi, 26, who grew up in west London.

British and U.S. authorities have said they knew the identity of "Jihadi John" but refused to make it public.

The British-based human rights group CAGE, which assists people who claim they are targeted by security officials, and friends said they believe the masked militant is Emwazi.

"There was an extremely strong resemblance," Asim Qureshi, a research director at CAGE, told the Post. "This is making me feel fairly certain that this is the same person."

Emwazi graduated from the University of Westminster-London in 2009 with a degree in computer programming, according to CAGE and the Post. The school said a person named Mohammed Emwazi left the university six years ago.

After graduation, he traveled to Tanzania for a safari with friends, but he was detained by authorities. He was taken to a police station, stripped to his underwear and held in a cell for 24 hours, according to a CAGE case file the group released Thursday.

Friends of Emwazi told the Post they believed Emwazi became radicalized because of the Tanzania incident.

He was deported to Amsterdam, where Dutch and British intelligence officials interrogated him about suspicions that he planned to join al-Shabab militants in Somalia. Shortly thereafter, his family sent him to live in Kuwait, fearing harassment from authorities.

Off-and-on interrogations continued for at least the next four years as Emwazi tried to travel between Kuwait and the United Kingdom. In the summer of 2010, officials blocked him from a flight to return to Kuwait. As a result, he lost his computer programming job there, and his fiancée called off their engagement because he couldn't get back to Kuwait to join her.

"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," he wrote CAGE in one of several e-mails he sent from June 2010 to January 2011. "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."

At least four of the e-mails expressed frustration over injustices toward Muslims. In 2013, Emwazi disappeared after he changed his name to Mohammed al-Ayan and tried to fly to Kuwait for a final time. Several months later, police told his family he had entered Syria.

"Jihadi John" — nicknamed that by hostages after the Beatles, along with three other British militants — began appearing in Islamic State videos in August. He's shown dressed in a black hood shrouding his face and brandishing a large knife in front of his victims, who are dressed in orange jumpsuits. He speaks in a British accent, threatening the United States and other Western nations, as well as other hostages held by the militants.

The videos show the man beheading several hostages, including Americans James Foley, Steven Sotloff and Abdul-Rahman Kassig.

The black-clad militant also appeared in videos showing the deaths of Britons David Haines and Alan Henning. Last month, he threatened the lives of Japanese hostages Haruna Yukawa and Kenji Goto, who were later killed.

Sotloff's family said Thursday they hope Emwazi is caught, and they feel "relieved" and "take comfort" that his identity was revealed, the BBC reported.

"We want to sit in a courtroom, watch him sentenced and see him sent to a super-max prison where he will spend the rest of his life in isolation," the family said.

Qureshi of CAGE described Emwazi as a "beautiful young man" during a news conference Thursday. Qureshi spoke out against the security measures that he says increase alienation among young Muslims in the United Kingdom.

"When are we going to finally learn that when we treat people as if they're outsiders, they will inevitably feel like outsiders, and they will look for belonging elsewhere?" Qureshi said. "Unless we arrest that narrative, we're just going to keep on seeing these things happen over and over again."

Not much is known about Emwazi's family, who declined a request for an interview with the Post, citing legal advice.

Media from around the world thronged around a property Thursday in Queen's Park in west London, the affluent area where Emwazi reportedly once lived.

Kirit Patel, who owns a nearby shop, said a customer told him Emwazi frequently shopped there. "He used to come in my shop regularly," Patel said. "The funny thing is, I can't remember his face — I can't remember him."

Leinwand Leger reported from Washington. Contributing: Katharine Lackey in McLean, Va.

Unmasked 'Jihadi John' felt harassed as Muslim - USA Today