According to Virginia Tech Police Chief Wendell Finchum, in November of 2005, he made contact with a female student through telephone calls and in person. The student called it "annoying" but declined to press charges.
In December 2005, Cho, an English major, sent instant messages to a second woman. He made no threats, Finchum said, but the student complained. Officers spoke to him at that time.
An acquaintance of Cho later contacted authorities concerned he might be suicidal, reports CBS News correspondent Cami McCormick. It was at that point he was taken to a mental health facility, not connected with the university, and later released.
Neither of the two women who complained about Cho in 2005 was among Monday's victims.
Cho's creative writing also disturbed his fellow teachers and some of his instructors.
In one of the plays he wrote, "there was a character where the father raped the son and the son was going to get back at him by killing him. And it became very brutal and graphic, and vulgar with the language, Sara Stevens, a Virginia Tech junior, said on CBS News' The Early Show (
But police and university officials offered no clues as to exactly what set off Cho Seung-Hui, a 23-year-old senior, on the deadliest shooting rampage in modern U.S. history.
"He was a loner, and we're having difficulty finding information about him," school spokesman Larry Hincker said.
"I had classes with him for three years, and he was known as being expressionless. He usually sat in class and I never heard him speak once in three years," Stevens, a former CBS intern, told Early Show co-anchor Harry Smith.
Other classmates say that on the first day of a British literature class last year, students took turns introducing themselves. When it was Cho Seung-Hui's turn to speak, he said nothing.
Click here for an interactive gallery of the victims.
The professor then looked at the sign-in sheet, and noticed that Cho had written a question mark instead of his name. The professor asked, "Is your name 'question mark?'" A classmate, Julie Poole, says Cho offered little response.
She says he then spent much of the class sitting in the back of the room, wearing a hat and seldom participating.
Poole says, "We just really knew him as the 'question-mark kid.'"
"He made me so nervous," Stevens said.
"I saw some poetry and he seemed to be very angry," Lucinda Roy, the English department's director of creative writing, said on The Early Show.
"I didn't feel that the students felt safe. They expressed to their faculty member some discomfort. And she said that she wasn't comfortable teaching him anymore," Roy said, so she tutored him herself.
She notified the university's counseling service and police department about Cho's behavior.
"They were very concerned, and their response was immediate in terms of trying to help. And then they seemed to hit a wall where there are all these legal issues," Roy said. "So unless he'd issued an actual threat, I was told he never said he was going to do harm to himself or someone else in an explicit way."
"We certainly are always sensitive to the issue of potential violence. It is very difficult to predict when what someone perceives as stalking is stalking, and then how it might translate into violence later. In general that's a very difficult thing to predict," Dr. Chris Flynn, director of the school's Cook Counseling Center, said at a news conference Wednesday. "Clearly if anyone had any warning about a violent incident, people would have stepped in and acted.:
Roy said she tried to get Cho to go to counseling voluntarily, but he resisted. News reports said that Cho may have been taking medication for depression.
"It was very strange. If you were to say something to him, it would take him about 10 to 20 seconds to say anything back, so there would be a long, long pause. And then when he did speak, he spoke only in a whisper, so you'd have to lean in to hear what he was saying," Roy added.
Karan Grewal shared a suite with Cho, and saw him just two hours before the massacre started.
"He came in. Looked as normal as usual, no expression on his face. Didn't seem angry, sad, anything. Just a normal look on his face. Just like the picture," Grewal told CBS News.
"He was my roommate," Joe Aust, 19, told The New York Times. "I didn't know him that well, though."
He added: "He was always really, really quiet and kind of weird, keeping to himself all the time," he said. "I tried to make conversation with him in August or so and he would just give one word answers and not try and carry on the conversation."
The emerging portrait of Cho, the quiet loner whose writing sent out alarms, is one that fits almost to a "T" a U.S. Secret Service profile of the, reports CBS News correspondent Wyatt Andrews.
In a study done after the Columbine massacre, the Secret Service studied 37 school shootings to learn the patterns of the school-aged assassins.
Most school attacks, the report said, come from loners with some kind of grievance, adds Andrews. "Many attackers felt bullied," ...or persecuted by others....and "more than half had revenge as a motive."
Cho — who arrived in the United States as boy from South Korea in 1992 and was raised in suburban Washington, D.C., where his parents worked at a dry cleaners — left a note that was found after the bloodbath.
A law enforcement official who read Cho's note described it Tuesday as a typed, eight-page rant against rich kids and religion. The official spoke on condition of anonymity because he was not authorized to speak to the media.
"You caused me to do this," the official quoted the note as saying.
Cho indicated in his letter that the end was near and that there was a deed to be done, the official said. He also expressed disappointment in his own religion, and made several references to Christianity, the official said.
The official said the letter was either found in Cho's dorm room or in his backpack. The backpack was found in the hallway of the classroom building where the shootings happened, and contained several rounds of ammunition, the official said.
Gun store owner John Markell, who sold Cho one of the weapons, described him to CBS News as a "nice, clean-cut college kid." He said he had no suspicions about Cho's purchase, but that it's "just terrible" to learn what the gun was used for.