The Columbine killings on April 20, 1999 were one of those pivotal events in American history.
Now Eric Harris and Dylan Klebold's notebooks have been published on the net, providing a valuable insight into their crazed ideas. (Warning: it's a long PDF file, and may take a bit to download, depending on your connection).
The lesson here? Take what people say seriously. They usually tell you what they're thinking. The problem is that we don't always listen. If you want to know what they think and what they're going to do, listen to them. Listen carefully.