In the west, he has become synonymous with the gruesome campaign of terror waged by the Islamic State, or ISIS.
Until now known only as "Jihadi John" due to his clear British accent, the masked jihadist killer featured in recent videos showing the execution of western hostages, wielding a knife while coldly justifying the subsequent act of murder as "revenge" for airstrikes against the terrorist group.
But today, reports claim that Jihadi John has been positively identified as Mohammed Emwazi, a Kuwaiti-born UK national who grew up in a "well to do" family in West London and graduated from a British university with a degree in computer programming.
According to the Washington Post, Emwazi traveled to Syria in 2012, where he later joined ISIS.
He first gained notoriety in August 2014, announcing - and then apparently carrying out - the beheading of American journalist James Foley.
Emwazi subsequently featured in the execution videos of US-Israeli journalist Steven Sotloff, British aid workers David Haines and Alan Henning, and American aid worker Peter Kassig.
In each of the videos he appeared clad entirely in black, brandishing a knife and mocking western audiences as his victims kneeled in front of him.
He also appeared in a video showing Japanese captives Haruna Yukawa and Kenji Goto, who were also later murdered by ISIS.
Mohammed Emwazi is in his mid-twenties, the report says, and cites former friends who described him as "polite" with "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."
He first came to the attention of British intelligence agencies in 2009, when he was arrested along with two other suspects upon arriving in Tanzania and promptly deported.
Britain's domestic intelligence agency MI5 accused him of attempting to travel to Somalia to join the Al Qaeda-linked Al Shabaab group - claims he denied in emails seen by the Post. But according to a former hostage Emwazi was "obsessed" with Somalia, and even forced his captives to watch Al Shabaab videos.
He was one of a trio of British ISIS terrorists who were responsible for western hostages at a prison in Idlib, Syria, and took part in the waterboarding of at least four captives, a former hostage said.
Following the revelations, British police have remained tight-lipped.
Commander Richard Walton, head of the Metropolitan Police's Counter Terrorism Command, told the BBC: "We have previously asked media outlets not to speculate about the details of our investigation on the basis that life is at risk.
"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."
US officials refused to comment on the Washington Post's report, as did Emwazi's family.