Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: A video has emerged believed to show ISIS executioner Jihadi John ranting on camera shortly after he arrived in Syria from the UK. The footage is of two bands of fanatical fighters joining forces in 2013 under the command of notorious terrorist Omar al-Shishani. At the front stands a man thought to be Mohammed Emwazi - the west Londoner last week unmasked as the killer of Western hostages in a series of videos which have shocked the world. A man thought to be Jihadi John proclaims allegiance to a terror group in footage filmed in 2013. The video is thought to be the first known appearance by the executioner now unmasked as Mohammed EmwaziVideo shows two bands of militants joining forces during Syrian warJihadi John believed to have been member of one of the groups involvedMan who looks like British-raised ISIS execution makes proclamationExperts highlight similarities between speaker and Jihadi JohnHe swears allegiance to group run by notorious terrorist Omar al-ShishaniAl-Shishani also appears in the footage, arm-in-arm with other fighters 
 Question:The group were not part of _ at the time the video was filmed, but joined the group in the months that followed.

ISIS