Among the volunteers who heeded the call of the Islamic State in 2014 to come to Syria and Iraq and join the army of the caliphate were two brothers from the Ruhr region of western Germany. They were identical twin brothers, Mark and Kevin K., who, unlike many of the recruits from abroad, were not raised Muslim in an immigrant community.

The blond, blue-eyed lads were born in 1989 in the town of Castrop-Rauxel, near the city of Dortmund, where the prevailing religious culture was Roman Catholic. Their parents were devoted to the twins, and their father was a police officer. The brothers appeared to have a conventional childhood, an upbringing dedicated to school and sports. According to an internet-accessible curriculum vitae obtained by the German magazine Der Spiegel, Kevin was the adventuresome one of the pair and in high school ventured abroad for a yearlong student exchange program in...

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