Harris and Klebold had planned to kill hundreds at Columbine using bombs.
After parking their cars, the duo met near Harris's car and armed two 20 pound (9 kg) propane bombs before entering the cafeteria a few minutes prior to the beginning of the A lunch shift. The youths placed the duffel bags containing the bombs—set to explode at approximately 11:17 a.m.—inside the cafeteria before returning to their separate vehicles to await the explosion, and to shoot survivors fleeing the building. Had the bombs exploded with full power, they would have killed or severely wounded all 488 students in the cafeteria and possibly collapsed the ceiling, dropping part of the library into the cafeteria.End Quote
Maybe I was not paying attention but what I remember of the aftermath was concern mostly about the availability of guns. Actually guns were secondary to the bombs that they were attempting to kill hundreds with.