Eric Harris and Dylan Klebod were the childhood friends turned mass murderers when they gunned down over 30 people during the massacre at Columbine High School in 1999.

At the time, it was the deadliest school shooting in U.S. history. Harris and Klebold committed suicide after killing 12 of their classmates.WSJ: A Nightmare Re-Examined (April 18, 2009)

• Harris and Klebold originally planned a massive bombing but they were not able to wire the timers for the propane bombs they had placed in the cafeteria. If the the bombs had detonated, they might have killed 600 people.

• In the original plan, the bomb would have gone off and then Harris and Klebold planned to gun down any remaining survivors. The duo also planed to place more bombs in their cars which they expected to detonate when crowds, reporters and rescue workers had gathered to the site.

• Harris and Klebold wanted everything to be captured on television to achieve their goal of a "nightmare so devastating and apocalyptic that the entire world would shudder at their power."Slate: The Depressive and the Psychopath (April 20, 2004)

• Three months after the Columbine shootings, the FBI held a summit in Leesburg, Virginia, which included world-renowned mental health experts. Among the professional experts were Michigan State University psychiatrist Dr. Frank Ochberg and Supervisory Special Agent Dwayne Fuselier, a clinical psychologist and the FBI's lead Columbine investigator.Slate: The Depressive and the Psychopath (April 20, 2004)

• Klebold was determined to be "hotheaded, but depressive and suicidal." He was the type of person who blamed himself for his problems. While Harris was described as "nice" by adults and some other kids, psychiatrists determined was a psychopath. Harris appeared well-spoken and polite but he was actually cold and calculating and enjoyed hurting others.Slate: The Depressive and the Psychopath (April 20, 2004)

• AP: Winfrey Cancels Program on Columbine Anniversary (April 20, 2009)