LONDON — For months he taunted, knife in hand, his voice slightly muffled behind the mask that became the grim symbol of Islamic State barbarism.

But when the identity of the killer known as “Jihadi John” was revealed Thursday, the profile that emerged was disturbingly familiar: a young man whose parents’ decision to immigrate to the West afforded him a comfortable life and an education, but who ultimately found identity and succor in extremist ideology.

His name is Mohammed Emwazi. And despite friends’ descriptions of a polite and quiet man not capable of violence, Emwazi’s links to extremist groups appear to have been longstanding, and he was well known to counterterrorism officials in London before he went to Syria.

He has become infamous there in the past six months as the unidentified man who has beheaded American, British and Japanese hostages. The Washington Post revealed Emwazi’s name and details of his life on its website Thursday morning, based on information from two close friends and others familiar with his case.

The BBC on Thursday posted an excerpt of what it said were 2011 court papers in which the British government described Emwazi, now 26, as a member of “a network of United Kingdom and East African based extremists which is involved in the provision of fund and equipment to Somalia for terrorism-related purposes.”

Raffaello Pantucci, a security analyst with the Royal United Services Institute, said Emwazi was probably part of an informal gang of young Arab men from West London who became fixated on going to Somalia to fight alongside Islamist militants there.

Many had criminal records and were involved in petty crime and drugs.

One member of the group, a Lebanese-born, British-educated man named Bilal al-Berjawi, ultimately became a senior figure in the Somalia-based al-Qaida affiliate al-Shabab. He was killed in a U.S. drone strike in 2012. Another close friend of Berjawi’s, Mohammed Sakr, who grew up in the same West London neighborhood, was killed in a drone attack a month later.

Emwazi, in the end, made his way to Syria, where 20,000 foreign fighters from 90 countries have flocked, according to U.S. figures.

British security officials would not comment about Emwazi on Thursday and would not confirm that he is Jihadi John. They cited an active police investigation. Cmdr. Richard Walton, head of the Metropolitan Police’s Counter Terrorism Command, said no public details of the ongoing probe of Emwazi will be released.

“We are not going to confirm the identity of anyone at this stage or give an update on the progress of this live counterterrorism investigation,” Walton said in a statement.

The Kuwaiti-born Emwazi appears to have left little trail on social media; his invisibility is so striking it appears his online presence may have been deliberately erased. Those who knew him in London say he had a penchant for wearing stylish clothes while adhering to the tenets of his Islamic faith. He had a beard and was mindful of making eye contact with women, friends said.

He was raised in a middle-class neighborhood in West London; he graduated from the University of Westminster in 2009 with a degree in computer science.

“If these allegations are true, we are shocked and sickened by the news,” said a statement from the University of Westminster. “Our thoughts are with the victims and their families.”

On Thursday, reporters descended on the West London address where Emwazi grew up, a well-to-do area with pockets of deprivation. One neighbor popped his head out the door and shouted expletives at the gathering of about 40 journalists. None of the other neighbors answered their doors. It was not immediately clear whether any of Emwazi’s relatives live at the address.

Worshipers at a mosque about 100 yards away expressed shock that the man known as Jihadi John once lived around the corner.

“It’s a big shock to me and everyone here,” said Khosru Khan, 32, a travel agent who was praying at the Harrow Road Jamme Mosque. “… The Muslim community has been demonized here and around the world. He seems to be some sort of bogeyman for the Muslim community. We don’t know who he is.”

Two close friends of Emwazi’s, who spoke on the condition of anonymity because of the sensitivity of the investigation, said they believe he started to radicalize after what Emwazi described at the time as a planned safari in Tanzania following his graduation.

Emwazi and two friends – a German convert to Islam named Omar and another man, Abu Talib – never made it on the trip. Once they landed in Dar es Salaam, in May 2009, they were detained by police and held overnight. It is unclear whether the reason for the detention was made clear to the three, but they were quickly deported.

Emwazi flew to Amsterdam, where he claimed that an officer from MI5, Britain’s domestic security agency, accused him of trying to reach Somalia, according to emails that he later sent to Asim Qureshi, research director at the rights group Cage.

Emwazi denied the accusation and claimed that MI5 representatives had tried to recruit him. But a former Islamic State hostage said that Jihadi John was obsessed with Somalia and made his captives watch videos about the militant group al-Shabab, which is allied with al-Qaida.

The Tanzania trip, experts said, was probably a ruse to obscure plans to link up with al-Shabab.

“It doesn’t ring true,” said Nick Lowles, chief executive of the anti-extremism group Hope Not Hate. “It’s a bit like the jihadis who say they’re going on a package tour to Turkey, then, once they’re there, shoot over the border to Syria.”

Berjawi and Sakr also had traveled to Kenya in February 2009 after telling relatives they planned to go on “a safari.” But they were deported back to the United Kingdom after a Kenyan hotel manager became suspicious. They returned to East Africa months later and ultimately made their way into Somalia.

Emwazi and his friends were allowed to return to Britain, where he met with Qureshi in the fall of 2009 to discuss what had happened. “Mohammed was quite incensed by his treatment, that he had been very unfairly treated,” Qureshi said.

Shortly afterward, Emwazi decided to move to his birthplace, Kuwait, where he landed a job working for a computer company, according to the emails he wrote to Qureshi. He came back to London twice, the second time to finalize his wedding plans to a woman in Kuwait.

In June 2010, however, counterterrorism officials in Britain detained him again – this time fingerprinting him and searching his belongings. When he tried to fly back to Kuwait the next day, he was prevented from doing so.

“I had a job waiting for me and marriage to get started,” he wrote in a June 2010 email to Qureshi. But now “I feel like a prisoner, only not in a cage, in London. A person imprisoned & controlled by security service men, stopping me from living my new life in my birthplace & country, Kuwait.”

In an interview, Qureshi said he last heard from Emwazi in January 2012, when Emwazi sent him an email seeking advice.

“This is a young man who was ready to exhaust every single kind of avenue within the machinery of the state to bring a change for his personal situation,” Qureshi said. In the end, Emwazi felt “actions were taken to criminalize him and he had no way to do something against these actions.”

Close friends of Emwazi’s also said his situation in London had made him desperate to leave the country. It is unclear exactly when he reached Syria or how.

One friend said he believed Emwazi wanted to travel to Saudi Arabia in 2012 to teach English but was unsuccessful. Soon afterward, the friend said, he was gone.

“He was upset and wanted to start a life elsewhere,” another friend said. “He at some stage reached the point where he was really just trying to find another way to get out.”

Once in Syria, Emwazi contacted his family. It is unclear what he told them about his activities there.

A former hostage who was debriefed by officials upon release said that Jihadi John was part of a team guarding Western captives at a prison in Idlib, Syria, in 2013. The hostages nicknamed the facility “the box.” Emwazi was joined by two other men with British accents, including one who was dubbed “George.” A former hostage said Emwazi participated in the waterboarding of four Western hostages.

Former hostages described George as the leader of the trio. Jihadi John, they said, was quiet and intelligent. “He was the most deliberate,” a former hostage, who spoke on the condition of anonymity, said in an interview.

Beginning in early 2014, the hostages were moved to a prison in the Syrian city of Raqqa, the Islamic State’s de facto capital, where they were visited often by the trio. They appeared to have taken on more powerful roles within the Islamic State.

About the same time, Qureshi said, he sent an email to Emwazi.

“I was wondering if you could send me your number,” he wrote. “Inshallah (God willing) it will be good to catch up.”

There was no response.