In a study done after the Columbine massacre, the Secret Service studied 37 school shootings to learn the patterns of the school-aged assassins, CBS News correspondent Wyatt Andrews reports.
Most school attacks, the report said, come from loners with some kind of grievance. "Many attackers felt bullied" or persecuted by others, the study concluded. "More than half had revenge as a motive."
That included Luke Woodham, who killed two classmates in Pearl, Miss., and who the Secret Service interviewed personally.
"People always picked on me and they always called me gay or stupid stuff like that," Woodham said.
Cho's sense of persecution — "you made me do this," he wrote — fits the pattern. So does his methodical planning: the way he chained the doors of Norris Hall.
The authors of the Secret Service report told Scott Pelley of 60 Minutes some attackers plan for more than two weeks and then look forward to the assault.
Former Secret Service agent Brian Vossekuil read from a chilling first-hand account.
"My assault on the school is the only thing I live for. I want to laugh at the pain I cause these fools," Vossekuil read. "Personally, I'll be glad to die because living my life has been the greatest hell I could imagine.'
And Cho, it turns out, had a flair for imagining hell.
A former classmate has blogged that students were afraid of Cho — that in one playwriting class, the plays Cho wrote: "had really twisted, macabre violence that used weapons I wouldn't even thought of. We students were talking to each other with serious worry about whether he could be a school shooter."
In one of those plays Cho's character wants revenge against an abusing stepfather.
"Must kill Dick. Must kill Dick. Dick must die," it reads.
In another play, Cho's teacher has to die in revenge for low grades.
"I wanna watch him bleed like the way he made us kids bleed," Cho wrote.
The Secret Service concluded that shooters typically tell someone else of their plan.
There's no evidence that Cho did that. However, almost every shooter telegraphs the fact he's having violent thoughts.
We now know this quiet loner was sending troubled signals all along.