Peter Kassig, American beheaded by Islamic State, may have been killed by a gunshot

According to new research done by closely examining the video, it appears the 26-year-old American aid worker beheaded by the Islamic State, Peter Kassig, may have been killed by a gunshot.

The analysis comes from the British think tank Quilliam and the US-based Terrorism Research and Analysis Consortium (TRAC), The Telegraph reports.

Surgeons and video experts have analyzed the video and noted things like the fact the actual killing is edited out, unlike the other beheading videos, and a wound they noticed above Kassig's left eye.

It is believed Kassig may have fought back when the militants attempted to behead him, and then they shot him and went forward with the beheading once he was already dead. The analysis also suggests certain aspects of the video were added in using an expensive editing program called Avid.

It is believed Jihadi John, the British Islamic State militant from the videos, may have been superimposed into the scene, meaning he was not present for Kassig's killing. The researchers noticed the shadows around him do not appear how they should.

"The analyst categorically believes that the shadows were added, and added poorly," Charlie Winter, a researcher at Quilliam, told The Telegraph.

Kassig's family released a letter in October they believe he wrote before he was killed that said he was "scared to die," but he was proud he helped "alleviate suffering" in the country.