“Jihadi John,” the masked, British-accented executioner featured in six of the ISIS videos showing victims being beheaded, was identified Thursday as London university graduate 26-year-old Mohammed Emwazi. Born in Kuwait, Emwazi was 6 years old when his parents moved to West London. He grew up in a prosperous family and graduated from the University of Westminster in computer siciences in 2009.

Emwazi’s identity was confirmed yesterday by two U.S. government sources. He reportedly had been known to security services for weeks, perhaps even months, but they had not identified him publicly as they hoped to capture him. In May 2009 he came to the attention of British intelligence services, who detained him when he and two friends landed in Tanzania on what he said was a safari taken in celebration of his university graduation. British officials suspected they three were headed to Somalia to fight with Al Shabaab, the Somalia-based terrorist group.

The man known as Jihadi John was first identified by The Washington Post website on Thursday, and his name confirmed by a senior British security official, who said that they had identified him some time before. The identification was also confirmed by a senior U.S. military intelligence official. Emwazi is alleged to be part of a group known as the “North London Boys,” who have links to Al Shabaab. In each video the executioner is dressed entirely in black, with all but the eyes covered.

Emwazi’s 2009 safari from Britain to Germany to Tanzania brought him to the attention of British security services. He traveled using the name Muhammad ibn Muazzam, but it is thought that he was starting down the road to radicalism even before this time. He was detained again upon his return from Tanzania and questioned by British and Dutch security officials.

Emwazi moved to Kuwait after his graduation to work for a computer company, returning to London at least twice. He was detained by British counter-terrorism officials in June 2010 and not allowed to return to Kuwait, which had refused to renew his visa. He was put on a terror watch list that prevented him from leaving Britain. He tried to travel to Saudi Arabia in 2012, reportedly to teach English, but was again stopped from leaving the country. Soon afterward he vanished, and is believed to have made it to Syria where he assumed the position of ISIS’ figurehead.

Asim Qureshi, research director at the British advocacy organization CAGE, which is opposed to what it calls the “war on terror,” had been in regular contact with Emwazi, but last heard from him in January 2012. Emwazi was in Idlib, Syria, by 2013, helping to guard Western hostages. He appeared in the first of the beheading videos of those hostages in August 2014.

Qureshi blamed Emwazi’s radicalization on the treatment he received by police officers who harassed him at airports, Kuwait cancelling his visa, and being “roughed up” by police on one occasion. He described the young man as kind and thoughtful, saying there appears to be no correlation between the person he remembers and the character in the beheading videos.

Security officials in Britain and the U.S. are said to be unhappy that Emwazi’s name has been released. They had avoided identifying him for fear that it would make him more difficult to catch. Jihadi John has been one of the world’s most hunted men since the first beheading video was released on social media.