The investigation determined that Harris and Klebold placed two 20-lb. propane tank bombs in the cafeteria the morning of April 20. Computer modeling substantiated by field testing indicated that had those two large 20-lb. propane bombs detonated with a cafeteria full of students, most would have been killed or severely injured by the resulting blasts and subsequent fireballs. There were approximately 488 students in the cafeteria at 11:17 a.m. on April 20, the time the bombs were set to detonate. In addition to the casualties caused by the explosions, the computer models demonstrated a strong likelihood of structural damage and partial collapse of the cafeteria and possible library above.
"Explosives expert Sid Woodcock said propane explosives of the type found are 'fairly sophisticated.' Just one bomb 'probably would have destroyed a good part of the school,' he said. 'These subjects were not only on a killing rampage,' said Jefferson County Sheriff John Stone, 'They were going to burn the school up.'
"Larry Bettendorf of the [B]ATF says the propane tank was rigged with a can of gasoline and an egg timer. He says the bomb was poorly designed and that may be why it never exploded. Or he said gunmen Eric Harris and Dylan Klebold may not have had time to set the timer. Bettendorf says the big question for now is how the gunmen got such a big bomb into the school"
An ABC News report from April 23rd outlines clearly that the propane kitchen bomb was a real bomb. The sheriffs office now claims bombs were never found in the kitchen. "The large bomb that authorities found in the school cafeteria kitchen Thursday was a 20-lb. device attached to a propane tank that had shrapnel, including nails, attached to it. The bomb was set with an egg timer, matches and a model rocket fuse, said Lawrence Bettendorf, an ATF agent. When the timer went off, it would trigger the match, which would light the fuse and ignite a gas can next to the propane tank. The bomb would have been enough to seriously damage the school building, but not level it, authorities said. A bomb squad removed and safely defused the device Thursday."