Sky News has spoken to a former member of Islamic State, who claims he witnessed the man known as Jihadi John murder Japanese hostage Kenji Goto.
The former translator said that the masked man in the video was indeed Mohammed Emwazi, the 26-year-old Londoner who has become the face of the world's most notorious terrorist organisation.
"Saleh" said Emwazi is employed as the chief killer of foreign hostages for Islamic State's media wing and that his murderous influence among the group is feared and respected.
Speaking from Turkey, where he fled to escape IS, Saleh explained in broken English how he was employed by the group to convince foreign hostages they were safe. He is the only person to admit seeing Emwazi kill.
He said: "When he killed Kenji Goto I live showed this [saw this] but not near, from a little [distance].
"After he was killed him, three or four person come and take over the body and put in a car. After that, John went on a different road.
"The big boss was there with them. Turkish man say 'put this camera there, change place there' but John [was] the big boss. All time, all time say to all 'fastly, fastly, fastly, we should finish'. So respect him. Only he talks orders – others do."
Saleh suggested Emwazi commanded such respect within the group because of his willingness to murder foreigners.
"Maybe because he use the knife," he said. "I cannot understand why he is so strong. One man can kill and all people will respect. A Syrian man anyone [in IS] can kill. But strangers [foreigners], only John."
Saleh claims foreign hostages captured and murdered by the group were subjected to numerous mock executions until the procedure became normal. It could explain why many hostages appeared calm in execution videos published online by the group.
He said: "He would say to me 'say to them, no problem, only video, we don't kill you, we want from your government [to] stop attacking Syria. We don't have any problem with you; you are only our visitors'.
"So they don't worry. Always I say to them 'don't worry, doesn't matter, nothing dangerous for you. But at the end I was sure [they would die]."
Emwazi grew up in northwest London and graduated from the University of Westminster with a degree in computer programming before travelling to Syria in 2013 and later joining IS.
Advocacy group CAGE said the Kuwaiti-born Briton was "extremely kind" and "extremely gentle", but had been harassed by the UK security services.
Research director Asim Qureshi said Emwazi's family was "in utter shock" that the "beautiful young man" had joined the militant group.