CU freshman Mike Twisselman was lifting weights in the Dal Ward Athletic Center when he learned his sister might be dead.

A friend came in at 12:45 p.m. and told him about two students dressed in black trench coats going into Twisselman's old school, Columbine High School, and firing indiscriminately. He told him about the horrific television reports. He told him about the carnage of what became the worst school shooting in American history -- leaving 15 dead and another 23 in the hospital.

"I knew that was right when lunch started," Twisselman said Tuesday night from his dorm room. "And I knew exactly where my sister was -- she was in the choir room. So I was like 'uh-oh.'"

Twisselman, a 1998 Columbine graduate, quickly changed clothes and bolted out of Dal Ward. He borrowed a friend's motorcycle, sped to Littleton and eventually arrived at Columbine Library, a few blocks away from the high school.

He still knew nothing about Katie, his sister.

"I felt kinda mad, kinda scared," said Twisselman, a 210-pound punter for the CU football team. "Sometimes I wanted to cry."

Across campus, graduates of Columbine stayed glued to their televisions for every bit of information on the crisis. Some, like Twisselman, went to Littleton to be with their families.

Students without personal connections to the Denver suburb monitored the situation on the Internet and on TVs they dragged into their classrooms. CU professors also watched with interest -- as parents understanding the horror of the situation, as researchers understanding the causes of the situation and as humans understanding the pain of the situation.

The bloodshed began at about 11 a.m. when two students walked into school and began spraying bullets and detonating bombs. Part of the so-called Trench Coat Mafia, a group of outcast students distinguished by their black trench coats, the duo apparently targeted ethnic minorities, athletes and anybody else they had a grudge against. By 1 p.m., the story made the national TV news.

Yet, Twisselman still couldn't find Katie.

When he finally found her, she ran up to him and gave him a hug. She had been holed up in the choir room for nearly an hour, but she had made it out alive, as had many of Twisselman's other friends.

"I went over and found my prom date from last year," he said. "I could just see it in her eyes. She was like, 'whoa.'"

He said two of his former teachers had been shot and four people he knew, although not personally, had been killed.

Other friends had close calls.

"I had a friend where a bullet hit his shoe and knocked his clog off," Twisselman said, too much in shock to realize the magnitude of what he was saying.

Delbert Elliot, a CU sociology professor and the director of the Center for Study and Prevention of Violence, said it is too early for anyone to absorb the full extent of the violence. Right now, he said, it is time to grieve for the victims and their families.

Priscilla Dann-Courtney, a clinical psychologist in Boulder, said it might be too soon to even grieve. She related the experience of surviving a tragedy like this to surviving war. In both cases, people have to disconnect themselves from the events to survive.

She said CU students with close connections to the high school might feel the same thing. But she said it is important to seek counseling as soon as possible.

"They (students) need to talk," she said. "The more they push it down, the more likely it is to return in some way. They're going to have to go through the grieving process, and that will initially be a little rough."

Students can receive free counseling at the CU Counseling and Psychological services at Willard (303-492-6766), the Psychiatry Clinic at Wardenburg (303-492-5654) and the Office of Victim Assistance at Wardenburg (303-492-8855).

Once the mourning is finished, Elliot said his center can begin asking why this happened. He said people who are marginalised and alienated are more likely to commit violence, although he was not sure if that was the case with the Columbine massacre.

He also said the stress of adolescence and the desire to be accepted could make marginalised juveniles more violent. However, Elliot said not all outcast teenagers respond with violence; some withdraw.

Elliot also said violence like this could be averted.

"Many times, you can see it coming," he said. "There are signs that it's coming and a part of the problem is that people don't take these things seriously."

Often, hollow threats of violence point to a more serious problem, Elliot said. And, he said, many schools aren't prepared to handle violence when it happens because they don't think it can happen to them.

Twisselman, meanwhile, could see the warning signs.

"I always wondered if somebody would lash out because so many people picked on each other," he said.

He said the Trench Coat Mafia was a favorite target of some athletes. But he never expected anything like this to happen at Columbine.

He remembers his high school as "a really cool place," where, with some exceptions, people were friendly.

That sentiment was shared by junior finance major Eric Jacobson, who graduated from Columbine in 1996.

"Never in a million years would I have thought something like what happened in Arkansas or Kentucky could happen somewhere like Columbine," he said. "It was not even in the realm of my perception."

A friend from high school called Jacobson at about 11 a.m. to tell him about the tragedy. Jacobson then rushed to the television, where he sat transfixed until his 17-year-old sister, a student at Columbine, called to tell him she was OK. She had gone to lunch off-campus shortly before the bloodshed started. When she tried to return to campus, she was stopped by police.

So for Jacobson, Tuesday's violence -- some of the worst violence in Colorado history -- did not take his sister, but it did take his vision of the idyllic town he grew up in.

"Columbine is in the suburbs on a park with a lake and a library -- it's been there for 25 years," he said. "It's just a typical suburb area with backyard barbecues and little league families. The neighbors will come over for dinner or to borrow sugar. ... I always felt fortunate to grow up in such a safe place. I definitely felt safe inside the high school."