A film purporting to show the beheading of James Foley was probably staged, according to a forensic science company that suggests the American journalist's execution may have been carried out off-camera.
A blip in the footage, released on social media last week by Islamic State militants, indicated that Foley may have had to repeat a line as he read a script to camera while kneeling in the sand with his hands tied.
The footage was most likely edited later using "slick post-production techniques", according to the analysis for The Times in London by an unnamed international forensic science company that has worked for police forces across Britain, the news organisation claimed.
There was also no blood on Foley's neck as the masked militant appeared to drew a knife across his neck.
The analysis suggested that the militant, who speaks with a London accent and is believed to be British rapper Abdel-Majed Abdel Bary, may have been a frontman for the execution, and not necessarily the killer.
"I think it has been staged," one of the forensic experts told the news organisation.
"My feeling is that the execution may have happened after the camera was stopped."
The Times said that the company did not question that Foley had been beheaded, but rather pointed out that camera trickery appeared to have been used.
Foley's words, which ran for one minute and 26 seconds, appeared to have been scripted, according to the analysis.
The militant was wearing a full-face mask, but the analysis concluded he was speaking the words in the footage.
The company also analysed the start of the supposed beheading.
"After enhancements, the knife can be seen to be drawn across the upper neck at least six times, with no blood evidence to the point the picture fades to black," the analysis said, noting that sounds allegedly made by Foley did not appear consistent with what would be expected.
The next shot is of Foley's decapitated body, with his severed and blood-covered head resting on his body.
The clip was released on YouTube last week as a "warning" to the US to stop its military involvement in Iraq.
The US recently began a bombing campaign in the north of Iraq to help turn back an advance of Islamic State fighters who had made advances against Kurdish-controlled areas and threatened to massacre religious minorities.