The hostage is kneeling on the ground with his hands bound. But the executioner hardly hesitates as he fires his pistol more than a dozen times, shooting his victim in the back of the head.
The face of the masked killer is never seen during the short propaganda film, but his voice is heard throughout it: he has a clear London accent.
The man who was filmed carrying out the execution in Raqqa in 2014 - the first known example of such a killing by a British citizen during the Syrian conflict - is a former bricklayer from west London, multiple sources have told ITV News.
Ali Almanasfi, who is 28-years-old, is believed to be the balaclava-wearing assassin, according to friends and officials familiar with the case.
The footage was posted online in the spring of 2014 by a group of British fighters in Syria called Rayat al Tawheed, or “Banner of God” – an affiliate of Islamic State group.
The post was accompanied by a message describing the victim, as one of “Bashar’s dogs”, referring to the Syrian president Bashar al-Assad.
British government officials will not comment publicly on the identity of the executioner in the film. Because he never identifies himself and does not show his face they cannot be certain who the killer is. But multiple sources in Syria and the UK have indicated they have little doubt.
Ali Almanasfi was a trainee bricklayer and the son of a Syrian bus driver. He was jailed for a violent assault but after serving his sentence decided to travel to Syria shortly after the conflict began.
Almansfi’s brother spoke to ITV News earlier this year, before we began investigating the allegations surrounding the execution film. He said his brother had become more interested in the Syrian conflict before vanishing without warning.
“He actually texted me off his English number saying: ‘Love you, bro. Take care, see you soon’. I understood straight away that meant ‘I’m going’.
"The next day I spoke to my sister and she said ‘Yeah, he didn't come back. He went last night to the mosque and he never came back’. I was like ‘whoa.’’
At home in west London, Almanasfi had already become friends with young men who had become obsessed with the conflict in Syria.
Two of them, Mohammed Emwazi and Alexe Kotey, would later become members of the ISIS execution gang known as ‘The Beatles’.
Another close friend, Imran Khawaja, travelled to Syria to join the same militant group as Almanasfi. He faked his own death during the conflict to try to return home undetected. He is now serving a 12-year prison sentence in London.
Almanasfi was assumed dead following an ambush by Syrian forces in 2013. State TV announced that “the terrorist” had been killed after taking a wrong turn at a government checkpoint near Idlib. But he is thought to have survived the attack before marrying and having children in Syria.
Last year, Almanasfi had his British citizenship revoked, like dozens of UK nationals who have travelled to Syria.
“To him, England’s history now, so the passport’s not going to make any difference to him,” said Almanasfi’s brother.
“He knows he’s going to live there and die there. Maybe now or twenty, thirty years’ time.”