The pair hoped that after setting off home-made explosives in the cafeteria at the busiest time of day, killing many hundreds of students, they would use their guns to shoot survivors as they fled from the school. Then, as police vehicles, ambulances, fire trucks, and reporters came to the school, bombs set in the boys' cars would detonate, killing the emergency personnel, media, and law enforcement officers; this original plan backfired when the explosives did not detonate
Harris and Klebold constructed a total of 99 improvised explosive devices of various designs and sizes. They sawed the barrels and butts off their shotguns to make them easier to conceal.[4] The perpetrators committed numerous felony violations of state and federal law, including the National Firearms Act and the Gun Control Act of 1968, even before the massacre began.