1998-99 is the senior year of both Eric Harris and Dylan Klebold. During the summer, both boys experimented with making pipe bombs. They also begin to record video tapes documenting their plans to attack the school. In the months leading up to the massacre, the boys had help purchasing guns, propane tanks, and causing plenty of mischief leading up to the horrific event.
- Summer 1998 - Eric and Dylan begin making pipe bombs. They may have done so previously but this is when they begin to use them more frequently.
- October-November 1998 - Robyn Anderson purchases three of the four guns for Eric and Dylan at a K-Mart in Colorado.
- December 10, 1998 - Eric and Dylan film Hitmen for Hire. What may seem like a typical homemade project, was simply a rehearsal for the worse to come.
- January 1999 - Dylan's homework about a "trenchcoat killing maniac" receives controversy from one of his teachers. Dylan says "it's just a story."
- February 1999 - Philip Duran purchases the last of four guns, the TEC-9, for Eric and Dylan.
- March 6, 1999 - Eric and Dylan practice firing their weapons at Rampart Range in the mountains of Colorado.
- March 15-April 20 - Eric and Dylan film the basement tapes, video tapes showing their plans of how far they've come in beginning to take down the people they hate.
- April 17, 1999 - Columbine's senior prom is held. Dylan Klebold attends, Eric Harris does not. "Somebody" changes the date on the posters (which read "it's coming") from 4/17/99 to 4/20/99.
- April 19, 1999 - Breakfast Run is filmed.
- April 20, 1999 - Day of the massacre, Columbine, Colorado's darkest hour.