The Basement Tapes
Beginning March 15, 1999, Eric Harris and Dylan Klebold made a series of videotapes specifically addressing the attack they planned, and expressing their reasons. Because many of the installments were filmed in Eric's basement bedroom, they came to be known as The Basement Tapes.
As of 2012, the tapes have never been released, or leaked, even in part. Any footage on the web purporting to be from The Basement Tapes is fake—or is part of other footage compiled by the killers. A narrative account of the most significant parts appears as "The Basement Tapes," chapter 50 of Columbine.
A Jeffco sheriff's officer wrote out a very detailed accounting of the entire contents of The Basement Tapes, which was released by Jeffco as evidence. It is scanned here: