The BBC and Washington Post reported on Thursday that the man who appeared wearing a ski mask in the execution videos had been identified as Mohammed Emwazi.
In his mid-20s, Emwazi is said to be from a well-to-do family and to have a degree in computer programming. He is said to have grown up in west London after moving to the UK with his parents from Kuwait at the age of six. The Post reported that he was believed to have traveled to Syria in 2012.
London's Metropolitan Police would not confirm the man's identity.
"We are not going to confirm the identity of anyone at this stage," Richard Walton, head of the police Counter Terrorism Command, said in a statement quoted by news agency AFP.
Reuters later on Thursday cited two unnamed US government sources as having confirmed the story.
Radicalized after detainment?
Mohammed had apparently been previously known to intelligence services, according to the BBC, but for security reasons they had not named him. He is also believed to have had connections to Somalia's terrorist al Qaeda-linked al-Shabab group.
Cage, a civil rights group which had contact with Mohammed over the past few years, released emails to the Washington Post which indicate that he may have become radicalized after being detained by authorities in Britain at an airport. They accused him of having links to terrorist organizations.
The organization's research director Asim Qureshi said it was highly possible that the masked Londoner in the videos was indeed Emwazi, though he added, there was no way he could be "100-percent certain."
"This case should trigger thinking about British domestic and foreign policy," Qureshi said. "What risk assessments, if any, have been made about British counter-terrorism policy and the key part it plays in radicalizing individuals?"
IS propagandist, executioner
"Jihadi John" first appeared in IS propaganda videos posted online in August of last year and is said to be responsible for the murders of three Americans and two Britons, all of whom were either journalists or aid workers.
The International Centre for the Study of Radicalisation at London's King's College university said it believed the identity "to be accurate and correct," AFP reported.
"The fact that 'Jihadi John' has been unveiled in this manner demonstrates that whatever efforts are made, the ability to mask one's identity is limited or in fact impossible, and their true identities will eventually be revealed," the research center said in a statement.
It said that his middle-class upbringing was indicative of how radicalization "is not something driven by poverty or social deprivation." The center added: "Ideology clearly plays a big role in motivating some men to participate."
British intelligence believes there are around 700 British militants working for IS in Syria and Iraq.
sb/msh (Reuters, AFP, AP)