It's another key piece of the puzzle and it comes from the killer's own hand.
Authorities in the U.S. are poring over the rambling writings of Cho Seung-Hui, the man behind Monday's stunning gun rampage at Virginia Tech University in Blacksburg.
What they're finding is both revealing and disturbing.
Essays and plays the 23-year-old English major wrote for his school assignments are showing a seriously disturbed individual, obsessed with death and a need for revenge.
In one play called "Richard McBeef", posted on the site
The Smoking Gun, Cho writes about a 13-year-old boy who accuses his own father of being a pedophile.
"I hate him," the main character says. "Must kill Dick. Must kill Dick. Dick must die. Kill Dick ... you don't think I can kill you Dick? Gotcha. Got one eye ... got the other eye."
The play ends with the boy trying to kill his dad, only to see the older man delivering a death blow to the teen.
Some of his compositions were so disturbing that his teachers confronted him on them. When professor Lucinda Roy asked Cho if anything was troubling him, he replied that his work was all 'satire' and not to be taken seriously.
But she didn't believe him and actually took the step of recommending counselling for him and alerting other staff members about the problem.
But nothing was ever done.
A note was found in Cho's room after the massacre that railed on about "rich kids", "deceitful charlatans" and "debauchery". It ends with the chilling words, "you caused me to do this."
Cops who found his lifeless body also discovered the last words Cho had ever written. "Ismael Ax" was scrawled in red ink on his arm, a bizarre phrase that may have had meaning only to the gunman.
There's growing evidence that Cho fit the pattern of the classic school shooter - a boy who had been repeatedly bullied and taunted by others.
A former friend remembers that Cho never talked in class and was so withdrawn, shy and silent, he was mocked by other students in his high school.
At one point, a bully offered him money if he'd just say a single word. But Cho refused, turning around and walking away from his tormentor.
On Monday, he finally let his gun speak for him. Thirty-two innocent people paid a terrible price for all that pent up anger that exploded into a bloodbath on campus and the worst shooting massacre in North American history.