On April 20, 1999, Columbine High School in Littleton, Colorado, was the scene of the most horrific act of school violence in U.S. history. The events that occurred that day were no accident. After a lengthy and methodical planning phase, Eric Harris, age 18, and Dylan Klebold, age 17, arrived at school ready to elicit fear and bring death to students and faculty. Armed with a small cache of firearms and a significant number of homemade bombs, the two carried out their mission.

Beginning at 11:10 a.m., the two parked their cars in the southwest parking lot, entered the school with duffel bags containing two 20-pound propane bombs, and planted them in the cafeteria. They returned to the parking lot and waited for the bombs to ...