Investigators are analyzing a notebook believed to be written by James Holmes, the accused gunman in Friday's Colorado movie theater shooting, which could be a roadmap to a massacre.
If it is, it would not be the first of its kind. Experts say detailed, meticulously written plans are often a hallmark of mass murderers.
"Universally, mass shooters [are] all about revenge," said Brad Garrett, a former FBI special agent and an ABC News analyst. "He wanted to pay society back for what he believed society had done to him. And I think the notebook will talk about that."
Holmes is reported to have walked into an Aurora, Colo., theater showing "The Dark Knight Rises" around midnight Friday dressed in riot gear and brandishing at least three weapons. He allegedly set off two smoke bombs before opening fire on the movie theater patrons with an assault rifle, shotgun and a handgun, killing 12 and wounding dozens of others.
When investigators first found the Holmes package on Monday in the mailroom at the University of Colorado, where Holmes recently dropped out as a neuroscience student, they were so concerned it -- like Holmes's apartment -- would be rigged with explosives that they sent in a robot to handle it.
Inside the notebook, they reportedly found plans for a massacre, including drawings of a stick-figure gunman mowing down his victims.
In America's overcrowded history of mass murder, nearly every perpetrator has left behind documentation.
Seung-Hui Cho, the student who killed 32 people and himself at Virginia Tech in Blacksburg, Va., in 2007, mailed photos, a letter and video clips of himself reciting a garbled rant at unnamed and perhaps unknowable wrongdoers.
"Thanks to you, I die like Jesus Christ, who inspired generations of the weak and defenseless people," he said in the video.
"You had 100 billion chances and ways to avoid today. But you decided to spill my blood," he said. "You forced me into a corner and gave me only one option. The decision was yours. Now you have blood on your hands that will never wash off."
In his letter, Cho even expressed admiration for fellow mass murderers Eric Harris and Dylan Klebold, the Columbine High School killers.
"We martyrs, like Eric and Dylan, will sacrifice our lives to ... you thousand folds for what you apostles of sin have done to us," Cho wrote.
Those Columbine seniors, who on April 20, 1999, killed 13 people in a shooting spree in Colorado, left voluminous diaries, diagrams of the school, and ominous videos before their killings and suicides.
More recently Jared Lee Loughner, accused in a shooting spree on Jan. 8, 2011, at a Tucson, Ariz., community event held by Rep. Gabrielle Giffords, that killed six people and wounded others, including Giffords, posted incomprehensible videos about his community college, calling it "my genocide school."
The video contained such strange narration as this: "If the student is unable to locate the external universe, then the student is unable to locate the internal universe."
The writings and videos of mass killers often seem bizarre and short on rational arguments, but they nevertheless may shed light on the motivations for the crimes.
"These attackers may be trying to be understood," said Marisa Randazzo, an expert on threat assessment and targeted violence. "Because at the time they carry out the attack they don't feel understood. This may be part of what is driving this personal desperation -- the feeling that they have no options left."
ABC News' Colleen Curry contributed to this report.