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Why the Columbine massacre? Book trailer outlines killers' motives
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This tight video by a filmmaker from the South Park team summarizes the killers' motives & Dave Cullen's Edgar-winning book COLUMBINE concisely in 3 minutes.

You will meet Eric Harris and Dylan Klebold and get a sense of what drove them and what they wrought. This is probably the easiest way to gauge whether Columbine is the right book for you.