"It could be a body double or a decoy," said Veryan Khan, the editorial director with the Terrorism Research and Analysis Consortium (Trac), which looked at the video for Quilliam, the British anti-extremist think-tank.
"I'm not trying to say I know the answer, but his shape and size are very similar."
The analysts believe parts may have been shot and reshot several times, and that it might have taken between four and six hours to film.
Although each jihadist seems to have an individual hostage to kill, careful study shows it is not always the same one. Hostages are exchanged.
The final section, with Kassig, is claimed to be filmed in Dabiq. Although the background is clearly Dabiq, Trac says it believes Jihadi John and the head were filmed elsewhere, and superimposed in editing.
Winter said it was possible the jihadists left out Kassig's body because "he struggled and his video became unusable for marketing purposes".
The analysts believe the editing is so sophisticated the full film may have been sent abroad to a professional studio.