The Virginia Tech shooter has been described as "cold," "abnormal" and "mentally impaired" -- by a member of his own family describing him as a young boy.
"Normally sons and mothers talk. There was none of that for them. He was very cold," Seung-hui Cho's great aunt Kim Yang-soon, 85, told AP Television News in South Korea. "When they went to the United States, they told them it was autism."
But while Cho was often silent, he would sometimes lash out. The Washington Post reported in today's editions that he would violently punch his older sister, Sun-kyung Cho, when he fought with her.
After the shootings, Cho's family members -- in both South Korea and the United States -- have expressed regret, grief and guilt over the actions of one of their own.
"Our family is so very sorry for my brother's unspeakable actions," Cho's sister said in a statement to The Associated Press on behalf of her family. "It is a terrible tragedy for all of us.
"We are humbled by this darkness," the statement said. "We feel hopeless, helpless and lost. This is someone that I grew up with and loved. Now I feel like I didn't know this person."
The statement said the family "never could have envisioned that he was capable of so much violence," and added, "He has made the world weep. We are living a nightmare."
Law enforcement officials told ABC News that investigators know how to reach Cho's parents for investigative purposes, though they are not at their Centreville, Va., home. One official said the parents are "doing fine."
FBI spokeswoman Debra Weierman called "totally erroneous" an earlier report by South Korea's Embassy that Cho's parents and sister were being moved from place to place by U.S. law enforcement.
"The Cho family is not under any type of law enforcement custody and protection," Weierman said.
In South Korea, where Cho, 23, spent his early childhood, his grandfather told one newspaper he'd rather be dead than witness his family's shame.
And Cho's uncle, identified only as Kim, expressed the family's sorrow to The Associated Press.
"I am devastated," the uncle said. "I don't know what I can tell the victims' families and the U.S. citizens. I sincerely apologize — as a family member."
In the United States, Sun-kyung Cho reportedly reached out to a friend at Princeton University, where she graduated in 2004, to apologize to fellow Koreans for reverberations from her brother's actions. The Rev. David Kim, director of the Manna Christian Fellowship group to which Sun-kyung Cho once belonged, told a Korean students' group she expressed feelings of guilt during a phone call, according to the report in the Daily Princetonian.
Sun-kyung Cho worked as a State Department contractor and lived at home with her parents.
The Cho family moved to the United States in 1992, where Sung and Hyang Cho, parents of Seung-hui Cho and Sun-kyung Cho, ran a dry-cleaning business in suburban Washington.
Some of Cho's classmates from Westfield High School in Centreville recalled a boy with odd, stiff mannerisms who rarely spoke. One former classmate said kids used to shout his name in the hall because he would never respond or make eye contact.
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.