ALTENA, Germany — When you ask locals why Dirk Denkhaus, a young firefighter trainee who had been considered neither dangerous nor political, broke into the attic of a refugee group house and tried to set it on fire, they will list the familiar issues.

This small riverside town is shrinking and its economy declining, they say, leaving young people bored and disillusioned. Though most here supported the mayor’s decision to accept an extra allotment of refugees, some found the influx disorienting. Fringe politics are on the rise.