'Jihadi John' is a Kuwaiti-born Londoner named Mohammed Emwazi

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, Mr. Emwazi’s links to extremist groups appear to have been long-standing, 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 Mr. 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 Mr. Emwazi, now 26, as a member of “a network of United Kingdom- and East African-based extremists which is involved in the provision of funds and equipment to Somalia for terrorism-related purposes.”

Raffaello Pantucci, a security analyst with the Royal United Services Institute, said Mr. Emwazi was probably part of an informal gang of young Arab men from West London who became fixated on traveling 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.

Mr. 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 Thursday about Mr. Emwazi 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 Mr. 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,” Cmdr. Walton said in a statement.

The Kuwaiti-born Mr. Emwazi appears to have left little trail on social media; his invisibility is so striking that it appears that 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.