The Islamic State militant known as 'Jihadi John' thought to have carried out the beheadings of five Western hostages, could have a body 'decoy' say terrorist experts.

The Terrorism Research and Analysis Consortium (TRAC) and the Quilliam Foundation have conducted an extensive investigation into the IS video showing the decapitated head of the American Peter Kassig.

The film footage shows a masked man clad in black standing before the head, speaking in a British accent: "This is Peter Edward Kassig, a US citizen, of your country; Peter who fought against the Muslims in Iraq, while serving as a soldier."

Veryan Khan, TRAC's editorial director, told Sky News that a masked militant who is roughly the same size and shape as Jihadi John is included for around a tenth of a second.

Although Jihadi John plays the central role, one other individual is wearing a balaclava, but only visible for a single "screen grab". That has led the analysts to suspect that he might be used as a double for the Isis executioner in order to protect him from assassination.

Khan suggested: "He could be a stand in, a double, he could be a decoy, or he just happens to have the same shape and size."

A body double could be used to confuse drones looking to bomb the militant, or to conceal the death or sickness of Jihadi John, Khan added.

Body decoys have been used previously by military dictators such as Saddam Hussein who used them to appear for him in dangerous situations. Adolf Hitler is known to have employed at least one double and possibly employed as many as six.

Analysts believe some sections of the beheading video may have been shot and reshot several times, and the entire footage may have taken between four and six hours to film.

The background is clearly Dabiq, although TRAC says it believes Jihadi John and the severed head were filmed in a different location, then superimposed using sophisticated video technology.

Khan said this was deduced from studying the shadows in the film and that the foreground and background do not match.

There are also suggestions that Abdul Rahman Kassig, who converted to Islam during his captivity, was in a violent struggle with his captors before he was killed as the American had a large bruise over his eye.

Charlie Winter, a researcher at Quilliam, said it was possible that the jihadists left out Mr Kassig's body because when they tried to make a film of his death "he struggled and then his video became unusable for marketing purposes".