Police in the US have released almost 1,000 pages of diary entries, school work and computer files written by the Columbine High School killers.
The teenagers appeared to have planned their attack for months
Among the papers are maps, drawings and lists of supplies needed before Eric Harris and Dylan Klebold opened fire.
The supplies listed included petrol, nails, and propane. Meticulous preparations for the deadly attack were also set out.
The teenagers killed 13 people and themselves on 20 April 1999.
In total police have released thousands of documents since the shootings in Littleton, Colorado, which remain the worst US high school attack.
But the latest information includes new details about the pair's planning and thoughts in the months leading up the attack.
"Once I finally start my killing, keep this in mind," Harris wrote in his diary in October 1998.
"There are probably about 100 people max in the school alone who I don't want to die, the rest MUST [expletive] DIE!"
His co-conspirator Dylan Klebold was no less direct: "Hell on Earth - ahh, my favourite," he wrote in Harris' 1998 yearbook.
"So many people need to die."
The documents include essays offering a chilling insight into the minds of the would-be killers.
Months before the shootings, Harris wrote a school essay about guns in school, another about "Nazi Culture", and one about convicted killer Charles Manson.
Klebold's writings predicted "more fun this school year than any others in the past".
The boys' drawings and writings had a violent theme
Detailed lists and approximate timings for the eventual shootings are also in the documents.
In Klebold's diary the events of 20 April 1999 appear to be planned in advance.
The documents, which join an estimated 20,000 documents already released by investigators, confirm a fascination with guns, violence, weapons and death by Harris and Klebold.
Some victims' families have long maintained that the boys' plans should have been unearthed in advance, and continue to call for full disclosure of all information relevant to the shootings.
Police refused to release videos and audiotapes made by the pair amid fears they could spark copycat killings.