(Jahi Chikwendiu | The Washington Post) Wreaths memorialize the victims of a mass shooting on Aug. 4, 2019, in Dayton, Ohio. Such mass shootings at the hands of disaffected young men — including an attack just hours earlier in El Paso, Texas, where a 21-year-old is accused of killing 22 people on Saturday — illustrate the difficulties of tracking people who have made dangerous threats.