According to journals, notes, and videos that Dylan Klebold and Eric Harris left to be discovered, Klebold had been thinking of committing suicide as early as 1997 and they both had begun thinking about a large massacre as early as April 1998 - a full year before the actual event.

By then, the two had already run into some trouble. On January 30, 1998, Klebold and Harris were arrested for breaking into a van. As part of their plea agreement, the two began a juvenile diversion program in April 1998. Since they were first-time offenders, this program allowed them to purge the event from their record if they could successfully complete the program. So, for eleven months, the two attended workshops, spoke to counselors, worked on volunteer projects, and convinced everyone that they were sincerely sorry about the break-in. However, during the entire time, Klebold and Harris were making plans for a large-scale massacre at their high school.

Klebold and Harris were angry teenagers. They were not only angry at athletes that made fun of them, or Christians, or blacks, as some people have reported; they basically hated everyone except for a handful of people. On the front page of Harris's journal, he wrote: "I hate the fucking world." Harris also wrote that he hates racists, martial arts experts, and people who brag about their cars. He stated:

You know what I hate? Star Wars fans: get a friggin life, you boring geeks. You know what I hate? People who mispronounce words, like 'acrost,' and 'pacific' for 'specific,' and 'expresso' instead of 'espresso.' You know what I hate? People who drive slow in the fast lane, God these people do not know how to drive. You know what I hate? The WB network!!!! Oh Jesus, Mary Mother of God Almighty, I hate that channel with all my heart and soul."1
Both Klebold and Harris were serious about acting out on this hate. As early as spring 1998, they wrote about killing and retaliation in each other's yearbooks, including an image of a man standing with a gun, surrounded by dead bodies, with the caption, "The only reason your [sic] still alive is because someone has decided to let you live."2

Klebold and Harris used the Internet to find recipes for pipe bombs and other explosives. They amassed an arsenal, which eventually included guns, knives, and 99 explosive devices.

Klebold and Harris wanted to kill as many people as possible, so they studied the influx of students in the cafeteria, noting that there would be over 500 students after 11:15 a.m. when the first lunch period began. They planned to plant propane bombs in the cafeteria timed to explode at 11:17 and then shoot any survivors as they came running out.

There is some discrepancy whether the original date planned for the massacre was to be April 19 or 20. April 19 was the anniversary for the Oklahoma City Bombing and April 20 was the 110th anniversary of Adolf Hitler's birthday. For whatever reason, April 20 was the date finally chosen.

* Though some claimed they were part of the Trench Coat Mafia, in truth, they were only friends with some of the group's members. The boys didn't usually wear trench coats to school; they did so only on April 20 to hide the weapons they were carrying as they walked across the parking lot.

Their rampage put schools on alert for "enemies lists" made by troubled students, but the enemies on their list had graduated from Columbine a year earlier. Contrary to early reports, Harris and Klebold weren't on antidepressant medication and didn't target jocks, blacks or Christians, police now say, citing the killers' journals and witness accounts. That story about a student being shot in the head after she said she believed in God? Never happened, the FBI says now.

A decade after Harris and Klebold made Columbine a synonym for rage, new information — including several books that analyze the tragedy through diaries, e-mails, appointment books, videotape, police affidavits and