No Warning Signs Here
From the AP

Citing unidentified sources, the Chicago Tribune reported Cho had recently set a fire in a dorm room and had stalked some women.
...
Some said that on the first day of a British literature class last year, the 30 or so students went around and introduced themselves. When it was Cho's turn, he didn't speak.

On the sign-in sheet where everyone else had written their names, Cho had written a question mark. "Is your name, `Question mark?'" classmate Julie Poole recalled the professor asking. The young man offered little response.

How can you set fire in a dorm room and still be, you know, a student?