The Basement Tapes
Scans of the police-evidence descriptions are below.
From March 15 to April 20, 1999, Eric Harris and Dylan Klebold made a series of videotapes specifically addressing the attack they planned, and expressing their reasons. Because most installments were filmed in Eric's basement (just outside his bedroom), they came to be known as The Basement Tapes.
In 2015, Jeffco announced that Sheriff Ted Mink had ordered all known copies of the tapes destroyed in early 2011, and that they are all gone. The tapes have never been released, or leaked, even in part. Any footage you find on the web purporting to be from The Basement Tapes is fake. But be aware that Jeffco did release some other video footage of Eric and Dylan. So if what you see looks like Eric and Dylan, that might be true, but just not The Basement Tapes.
However, the tapes have been heavily documented:
A Jeffco sheriff's officer wrote out a very detailed accounting of the entire contents, with numerous direct quotes. The official document runs ten pages, single-spaced. I have scanned all ten pages below.
The tapes were shown to a small group of reporters one time (with separate showings to Time, Rocky Mountain News, and the families). All those news organizations printed stories the next day, summarizing and quoting.*
The head of the FBI investigation, Supervisory Special Agent Dwayne Fuselier—a clinical psychologist with a PhD—watched every frame of the tapes dozens of times, analyzing them meticulously. Lead Investigator Kate Battan also watched them repeatedly.
I created detailed spreadsheets compiling all known published data on the tapes (including all the above), to reconstruct most of the contents. I then reviewed them at length with Dr. Fuselier, and with Lead Investigator Kate Battan. I have not published the spreadsheets, but a narrative account of the most significant parts appears as "The Basement Tapes," Chapter 50 of Columbine.
* Note: I get asked a lot, so I'll explain here that I was excluded from the single media showing, apparently as retribution for publishing the first leaked passages from Eric Harris' journal a few months earlier. I considered driving out to Golden when word broke that the Rocky reporter had seen them, but I didn't have a cell phone yet, so I stayed by my phone, repeatedly calling PIO Steve Davis, who usually returned my calls promptly. Steve called shortly before the viewing ended to gleefully let me know I had missed it. (He/they had been furious at me for the leaks, and also for publishing an exclusive interview with Kate Battan.) He said not to worry--they would do another showing in a few days, so that national media could fly in. I believe he meant that. However, his superiors quickly decided to withhold the tapes again, and never showed them again. So I've never seen them.
Here is the Jeffco sheriff's officer's account of The Basement Tapes: