In October, 2014, Mohammed Hamzah Khan, a nineteen-year-old engineering student from suburban Chicago, was arrested at O’Hare Airport, where he planned to board a plane to Vienna and then continue to Istanbul, on the way to join ISIS fighters in Syria. Khan’s story remained in the local news for months, and eventually became the opening of the terrorism analyst Peter Bergen’s book “United States of Jihad,” in part because he seemed an unlikely radical: high-achieving, assimilated, a good kid. But there was another interesting feature to the Khan case: his seventeen-year-old sister and sixteen-year-old brother had attempted to travel with him. Khan had carefully saved more than twenty-eight hundred dollars from his part-time job so that he could buy three tickets, not one. The letters that the Khan children had left for their parents, describing their motives, echoed one another’s language. Their parents, immigrants from India, said afterward that they were shocked, and that they had not noticed anything unusual about their son’s behavior, with the exception of an inordinate amount of time spent on his phone, which the parents did not monitor as closely as their one computer. Perhaps one reason that the Khan parents missed the evidence of their children’s radicalization was that it must have taken place partly in the most mundane of settings, in their rooms and cars, with one another.

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