By Masha Gessen, from The Brothers: The Road to an American Tragedy, out last month from Riverhead Books. Gessen is the author of several books, including The Man Without a Face: The Unlikely Rise of Vladimir Putin. Her most recent article for Harper’s Magazine, “Northern Exposure,” appeared in the June 2014 issue.

In the immediate aftermath of the 2013 Boston Marathon bombing, police scanned the crowd for people who looked suspicious, which is to say Muslim, which is to say darker than Boston-white. A twenty-year-old student of English from Saudi Arabia named Abdulrahman Ali Alharbi was among the dozens of…