IN A SOCIETY that supposedly has erected so many barriers to protect children from harm, a clearly troubled young man named Cho Seung-Hui managed to slip through every crack.

When two women on the Virginia Tech campus claimed in late 2005 that classmate Cho was stalking them and sending disturbing instant messages, both police and college authorities looked into the matter, but nothing initially came of it.

A month later, a Virginia district court ruled that Cho was mentally ill and "an imminent danger to self or others," but after further examination the creative-writing student was set free.

And when the 23-year-old showed up in a Roanoke gun shop last month to pick up a Glock 19 9 mm semiautomatic pistol, nothing turned up on his mandatory background check in Virginia State Police computer records.

That was little more than a month before Cho went on the nation's worst-ever shooting rampage, killing 32 people before turning one of his two handguns on himself - and ripping the heart out of a Blue Ridge Mountains community in the process.

The killings happened even though Cho's strange and anti-social behavior was so well-known in his corner of the Virginia Tech community that some students thought of him the second they learned of the mass shooting.

Last night, the nation got a firsthand glimpse of some of the depravity that Cho had exhibited on the Blacksburg campus - in the form of a bizarre series of pictures and digital videos that he had mailed to NBC News, apparently between his early-morning shooting of two students and his later attack, which killed 30 other people.

In the video and in photos, Cho, who immigrated to the U.S. from South Korea in 1992 - frequently brandishes two handguns, most likely the ones he used to carry out Monday's deadly assault.

"I didn't have to do this. I could have left. I could have fled. But no, I will no longer run. It's not for me. For my children, for my brothers and sisters that you f---; I did it for them," Cho says on one of the profanity-laced videos that was aired last night on the "NBC Nightly News." "When the time came, I did it. I had to."

Some of Cho's diatribe is a verbal assault against the rich. "You had everything you wanted," he says. "Your Mercedes wasn't enough, you brats. Your golden necklaces weren't enough, you snobs. Your trust fund wasn't enough. Your vodka and Cognac weren't enough. All your debaucheries weren't enough. Those weren't enough to fulfill your hedonistic needs. You had everything."

The shocking videos sent to NBC were another blow to a nation that was anxious to jump-start the healing process from the latest explosion of gun violence. And for a time, the footage eclipsed the question that lingered in the minds of many: Could Cho have been stopped before he became a killer?

Experts last night were not so sure the rampage could have been prevented, noting that despite Cho's history of strange behavior, he had never been accused of committing a violent act.

"It is true that people like Mr. Cho and the Columbine shooters exhibited some aberrant behaviors that, with 20-20 hindsight, might have tipped off sensitive observers," Dr. Jeff Victoroff, an associate professor at the University of Southern California and an expert on human aggression and the neurobiology of violence, said on the WebMD site last night.

"But we don't usually attend to those warning signs because they are so common among adolescents," he added. "We will never be altogether safe from such people."

In 2002, the Secret Service and U.S Department of Education published a lengthy report looking at some 37 school-shooting episodes dating from the 1970s and looking for ways to prevent them.

What the report found is frighteningly similar to what emerged in the Virginia Tech massacre, that most attackers engaged in some behavior before the incident that caused others concern or indicated a need for help. But it also found there is no useful or accurate profile of students who engaged in targeted school violence.

During Cho's time at Virginia Tech, the initial concerns about the student focused on his treatment of women - particularly in a class taught by poet Nikki Giovanni, who spoke so eloquently at Tuesday's memorial service.

About five weeks into the fall 2005 semester, students told Giovanni that Cho was taking photographs of their legs and knees under the desks with his cell phone. She told him that was inappropriate and to stop, but the damage was already done.

Female students refused to come to class, submitting their work by computer instead. As for Cho, he was not adding anything to the classroom atmosphere, only detracting. Eventually, Giovanni stood up to Cho and had him removed from the class, but efforts to help him fell short.

ABC News reported last night that Cho was evaluated at a psychiatric hospital near Virginia Tech where he was taken by police in December 2005, after two female schoolmates said they received threatening messages from him, and after police and school officials became concerned that he might be suicidal.

According to the "temporary detention order" obtained by ABC News, psychologist Roy Crouse found Cho's "affect is flat and mood is depressed.

"He denies suicidal ideation. He does not acknowledge symptoms of a thought disorder," Crouse wrote. "His insight and judgment are normal."

It is unclear how long Cho stayed at Carilion St. Albans, a private psychiatric hospital, although court papers indicate he was free to leave as of Dec. 14. Virginia Tech spokesman Larry Hincker said Cho had been continually enrolled at Tech and never took a leave of absence.

A spokesman for Carilion St. Albans would not comment.

Though the incidents with the two women did not result in criminal charges, police referred Cho to the university's disciplinary system, Virginia Tech Police Chief Wendell Flinchum said. But Ed Spencer, assistant vice president of student affairs, would not comment on any disciplinary proceedings, saying federal law protects students' medical privacy even after death.

Some students refused to second-guess the university.

"Who would've woken up in the morning and said, 'Maybe this student who's just troubled is really going to do something this horrific?' " said Elizabeth Hart, a communications major and a spokeswoman for the student government.

Meanwhile, at least one of the major mysteries surrounding Monday's shooting was resolved yesterday: What Cho was doing in the two hours between the first shooting inside a dormitory and his rampage in an engineering building?

Cho's package arrived at NBC headquarters in New York on Tuesday and was opened yesterday. It bore a Postal Service time stamp showing that it had been mailed at a Blacksburg post office at 9:01 a.m. Monday, about an hour and 45 minutes after Cho first opened fire.

NBC said the package contained a rambling and often-incoherent 1,800-word video manifesto on CD, plus 43 photos, 11 of them showing him aiming handguns at the camera.

"You had a hundred billion chances and ways to have avoided today," Cho said in a harsh monotone. "But you decided to spill my blood. You forced me into a corner and gave me only one option." *

The Associated Press contributed to this article.