First in a series on the Columbine shootings, 10 years later.
At his worst after he survived the Columbine shootings in Littleton, Colo., chemistry teacher Kent Friesen would become physically ill, often throwing up. Now, a decade later, certain triggers can still set off crying jags.
Friesen remembers running past the bodies of two students in the science wing that day as he tried to get help for girls' basketball coach Dave Sanders, who was gunned down and eventually bled to death in the melee.
"I knew he was going to die," said Friesen, who has had to unearth the trauma again as the victims, the country -- and the media -- brace for the 10th anniversary of the most iconic of all school shootings.
On April 20, 1999, Columbine High School seniors Eric Harris and Dylan Klebold, wielding automatic weapons, slaughtered 12 students and one teacher. Another 24 were injured in the 45-minute rampage.
Though Columbine is now a decade old -- and a larger shooting at Virginia Tech that left 32 dead followed in 2007 -- the wounds of this tragedy are still raw.
A number of books have tried to dissect the motives of the killers and the truth behind the event that played out on television screens across America, but no studies have ever examined the psychological effects of the massacre on those left behind, according to Dr. Frank Ochberg, a former FBI psychiatrist who guided the counseling teams in the aftermath of Columbine.
"There were relatively few who were right at the heart of [the shootings] and were direct post-traumatic stress syndrome candidates," Ochberg told ABCNews.com. "But there were relatively many for whom Columbine was their Gettysburg."
"It changed them, and if they were young, they grew up fast," he said. "Within that group, some are better and some are worse."
Friesen had been holed down in his classroom for three hours that day, with the incessant blaring of the school alarms, exploding pipe bombs and students' screams.
"A gun and holster is the worst one for me," said the 58-year-old, who has retired from teaching and lives in Littleton. "But you live with it. You learn to cope."
That takes him back to the frenetic day when Special Weapons and Tactics, or SWAT, teams, as confused as those already inside the school, roughed up Friesen, initially believing he was a suspect.
He said the easiest part of dealing with the shootings was getting free counseling for his post-traumatic stress disorder, but many never sought help.
"She saved my life," he said of his counselor. "It was the worst thing I have ever gone through," Friesen told ABCNews.com. "Post-traumatic stress can happen to anybody. It's mind-boggling to know that people still don't believe in it. It's real, and it's one of those things that just won't go away unless you get help."
According to Ochberg, who specializes in treating post-traumatic stress and the effects of violence, "The healthiest thing, hard as it is, is to grieve a loss.
"Grief is good and normal," said Ochberg, the former associate director of the National Institute of Mental Health.
But when distress becomes disorder, post-traumatic stress -- or what was once called "shell shock" or "battle fatigue," is often diagnosed.