On an icy Friday morning, Mahad Omar watched armed federal agents run down the street and tackle one of his neighbors to the ground. They handcuffed the man and put him inside a black-tinted S.U.V. Mr. Omar, 28, an Uber driver, immigrated to Minneapolis from Somalia two decades ago. He had never imagined seeing something like that in his community. “Minneapolis is a great city,” Mr. Omar said after the agents had left and residents emerged from their homes to discuss in hushed voices what they had seen from their yards and windows. Several women wept. “It’s never been like...