 Complete the following phrase:  By that time, Klebold had already arrived at the school in a separate car, and the two boys left two gym bags, each containing a 20-pound propane bomb, inside the cafeteria. Their original plans indicated that when these bombs detonated, Harris and Klebold would be camped out by their cars and shoot, stab and throw bombs at survivors of the initial explosion as they ran out of the school. At noon, this would be followed by bombs set up in the pairs cars detonating, killing first responders and other personnel. When these devices failed to detonate, Harris and Klebold launched a shooting attack against their classmates and teachers. It was the deadliest attack ever perpetrated at an American high school until the 