At least one of Cho's victims at Virginia Tech, Ross Alamaddine, sat a few feet from Cho in English class as they studied horror films and literature. Alamaddine tried repeatedly to talk to Cho but was rebuffed every time, ABC News has learned.
For the last year and a half, as many as eight of Cho's professors at Virginia Tech had formed what amounted to a task force to discuss how to handle him. According to The New York Times, they gathered at least twice, reaching out to university leaders as early as last September, telling officials they believed Cho was in trouble.
Sarah DeFluri, Cho's classmate at Virginia Tech and at Westfield High School, said she never thought the shy boy was capable of a massacre, but she and her classmates always considered him odd.
"Before we found out who the shooter was, my roommates and I were watching the news and we were all, 'What if it were Seung?'" she said. "But then we thought it was too weird -- we're just accusing this poor guy. He's too shy to do anything like that. But his name definitely came up before we knew."
ABC News' Marcus Baram, Dan Harris, Kirit Radia, Jack Date, John Donvan and Elisa Roupenian contributed to this report.