On the morning of April 20, 1999, Eric Harris, 18, and Dylan Klebold, 17, walked into Columbine High School, outside Denver, and shot to death 12 fellow students and a teacher. They then killed themselves. Their actions were the result of a yearlong plot that included plans to blow up the school and kill as many as 500 people.

Ten students, including Mr. Harris and Mr. Klebold, were found dead in the Columbine library. The teacher died of his wounds in a classroom. Two more students were found dead outside the school.

The Columbine rampage was on record as the deadliest school shooting in United States history until a student killed 32 people, and himself, at Virginia Tech on April 16, 2007.