Police are investigating why a teenage girl brought a handgun to a small Christian school in Madison, Wisconsin and opened fire, fatally shooting a student and a teacher and wounding several others — casting the tight-knit religious community into mourning just days before the school was set to close for Christmas.

Data indicate a familiar story in what underlies school shootings: Most perpetrators, male or female, are suicidal, in crisis and driven by a mix of hopelessness and rage.