On Saturday night, after Cole Tomas Allen’s alleged attempt to assassinate President Trump and administration officials at the White House Correspondents’ Dinner, a familiar ritual began on the internet: compiling a portrait of the shooter, based on the digital breadcrumbs of his online life. Over the past decade, this trail has frequently led to a similar place. The suspect in many cases turned out to be a white man radicalized by spending time in the internet’s dark crevices.