shadows cast by Jihadi John suggests he was superimposed into shot using £200,000 editing software favoured by Hollywood

Extraordinary new research has emerged suggesting Peter Kassig's beheading was faked by ISIS, and that the U.S. aid worker was killed by a gunshot, not decapitated by Jihadi John as implied.

Medical experts examining an ISIS propaganda video in which Mr Kassig's severed head is seen at the feet of ISIS' British executioner in chief, have concluded that he is likely to have been shot dead.

The claims add further weight to speculation over why Mr Kassig's full body was not shown in the video. Unlike ISIS' previous sickening filmed murders, he did not speak directly to camera before being killed and his body was not shown after the murder.

A popular theory is that Mr Kassig was killed before the video was shot because he did not cooperate with the jihadists - either refusing to give a final speech on camera or possibly even fighting back while the murder was taking place.

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Claims: New research has emerged suggesting Peter Kassig's beheading was faked by ISIS, and that the U.S. aid worker (pictured) was killed by a gunshot, not decapitated by Jihadi John as implied

Faked: Shadows cast by Jihadi John do not match the alignment of the sun and suggest the digital editing programme Avid - which costs around £200,000 - had been used to superimpose him into shots

Detailed examination of the murder video - in which 22 Syrian soldiers were also filmed being brutally beheaded by a group of unmasked ISIS killers - suggests that Mr Kassig was killed with a single gunshot wound to the head before being decapitated.

Charlie Winter, a researcher at Quilliam - counter-extremism think tank that will be publishing the analysis this week - told The Telegraph: 'We have had a senior face and neck surgeon independently look at some of the screenshots we took.'

'His initial assessment was that a contusion above Mr Kassig's left eye was a gunshot wound. Closer analysis of the wound reinforced that initial assessment, leading him to believe that the wound was probably the cause of death, rather than beheading,' he added.

Digital analysis of the film also suggests that Jihadi John may have been superimposed in the frame where he is seen standing over Mr Kassig's severed head. The raises the question of whether Mr Kassig was murdered in an entirely different location to the Syrian soldiers.

It is understood that shadows supposedly cast by Jihadi John do not match up with the alignment of the sun and that the digital editing programme Avid - which costs around £200,000 and is used in Hollywood - had been used to add shadows, albeit in an amateur fashion.

Sickening: The latest ISIS video showed a group of Syrian soldiers being marched along a desert road in Dabiq and brutally murdered by a large number of unmasked militants

Unusual: Peter Kassig is pictured while working as an aid worker in Syria before his capture. His murder is not actually shown in the film and he does not deliver a final message as other Western hostages have

Another alternative claim emerged just days after Mr Kassig's death and suggested that the aid worker could have been accidentally killed in a U.S.-led airstrike.

Speaking over Skype from a hiding place near the Turkish border, the head of the anti-ISIS resistance group Raqqa is Being Slaughtered Silently claimed there are reports that Mr Kassig died on November 5, when coalition fighter planes and drones pounded Tel-Abyad in northern Syria.

The resistance leader, a 22 year old medical student who uses the nom de guerre Abu Ibrahim Raqqawi, said in an interview: 'I think ISIS didn't execute Peter. An ISIS soldier told me this morning that Peter died in an airstrike on 5th November in Tel-Abyad and that is why they didn't show a video of the execution.

'The soldier was one of the first who arrived on the scene to secure it after an airstrike hit the Islamic State court building in Tel-Abyad. He said there was immediate high security and nobody was permitted entry to the building, not even ISIS fighters. Then two bodies were taken out of the building and put into a speeding car.'