Channel 4 said it had found the man behind the @ShamiWitness account by first finding the name of a former Twitter account he had run, and then using that name to search other social media sites until it found a Facebook page belonging to a man named Mehdi.

Asked in a telephone interview by the channel whether he supported the Islamic State’s methods, which have included mass executions of captive soldiers and videotaped beheadings of journalists and aid workers, the channel said Mr. Biswas replied, “Mostly.”