On April 20, 1999, two students at Columbine High School near Littleton, Colorado, went on a shooting spree that resulted in the deaths of twelve students and one teacher, and the injury of two dozen others, before taking their own lives. It is the most infamous school shooting in American history.

The two Columbine shooters—Eric Harris and Dylan Klebold—were both seniors at the time and by all appearances planned the shooting far in advance. As early as 1997, Harris was posting death threats against other students on a website he created, as well as pictures and information about his experimentations with creating homemade bombs. Over a year before the shootings, the two were arrested for stealing computer equipment out of a parked van, but they avoided serious punishment by agreeing to participate in both counseling and community service programs. At around this time, the...