Columbine killer's mother Sue Klebold on relationship with her son, warning signs she missed, what she went through after the tragedy

— -- On April 20, 1999, Dylan Klebold and Eric Harris walked into Columbine High School, carrying weapons and homemade bombs, and began slaughtering their classmates. They killed 12 of their fellow students and a teacher, and wounded 24 more people before turning the guns on themselves.

The Columbine massacre not only shocked the community of Littleton, Colorado, but stunned the nation and forever changed how school administrations and law enforcement handle school shootings.

Sue Klebold, Dylan Klebold's mother, believed that like many parents, she was sure she would have known if something were wrong with her son -- but all that changed after the tragedy.

“Before Columbine happened, I would have been one of those parents,” Klebold told Diane Sawyer in an exclusive interview for a special edition of "20/20." “I think we like to believe that our love and our understanding is protective, and that ‘if anything were wrong with my kids, I would know,’ but I didn’t know, and I wasn’t able to stop him from hurting other people. I wasn’t able to stop his hurting himself and it’s very hard to live with that.”

After Columbine: Experts Offer Tips for How to Talk to a Troubled Child

Resources for Suicide Prevention, Mental Illness Concerns, Helping Parents Cope

'A Mother's Reckoning' by Sue Klebold

FULL COVERAGE: Diane Sawyer's Exclusive with Sue Klebold

In Sawyer’s exclusive interview, Klebold talks about her relationship with her son, the warning signs she missed and the grief and shame she has grappled with for 17 years. This special edition of "20/20" also examined teenage mental health issues, included expert interviews about how to spot and help a troubled child and how other school shootings were prevented.

This was Klebold’s first television interview since the Columbine attacks. The interview coincides with the release of her new memoir, “A Mother’s Reckoning: Living in the Aftermath of Tragedy,” out on Feb. 15. Klebold said she is donating her book profits towards research and charitable foundations focusing on mental health issues.

She has decided to come forward in hopes that sharing her story will help other parents and caregivers possibly learn from her experience and recognize when a child might be in crisis.

“I want people to be aware that things can seem awfully right when things are terribly, terribly wrong,” she said.

Below are a few things Sue Klebold spoke about in the interview:

What Sue Klebold Wants to Say to Columbine Survivors and Victims' Families

“The one thing, of course, that I want to say is I am so sorry for what my son did, yet I know that just saying ‘I’m sorry’ is such an inadequate response to all this suffering,” Klebold said. “There is never a day that goes by where I don’t think of the people that Dylan harmed.”

“You use the word ‘harmed,’” Sawyer observed.

“I think it’s easier for me to say harmed than killed,” Klebold continued. “And it’s still hard for me after all this time... it is very hard to live with the fact that someone you loved and raised has brutally killed people in such a horrific way.”

ABC News attempted to speak to every Columbine family prior to airing our report. Click here for more.

What Sue Klebold Remembers From the Day of the Columbine Tragedy

On April 20, 1999, Klebold was working at an office where she helped disabled college students when she received a call from her husband Tom, a geophysicist who worked from home. He had called to say there was an emergency.

“His voice sounded horrible, jagged and breathless… ‘something terrible is going on at the school,’” Klebold said.

Her husband told her two killers wearing trench coats were shooting students at Columbine High School, and that one of Dylan’s friends had called because he was worried that Dylan might be involved.

"You always think somebody's making a mistake," Klebold said. “My first thought was Dylan may be in danger, you know, ‘who are these people that are hurting people?’”

She raced home, and after she got there, she learned her son was believed to be one of the shooters.

“The police were there and helicopters were going over,” Klebold said. “And I remember thinking, ‘if this is true, if Dylan is really hurting people, he has to-- somehow he has to be stopped.’ And at that moment, I prayed that he would die, that-- ‘God, stop this, just make it stop. Don’t let him hurt anybody.’”

Klebold would learn later that day that Dylan was dead, but it was the beginning of a long quest to go back over her life with a magnifying glass, looking for her son’s descent and what she missed.

"The last moments of his life were spent in violence, sadism, you know, he was cruel and hateful and I have to own that," she said.

Today, she and her husband Tom are divorced and he has chosen not to speak publicly.

What Sue Klebold Remembers as the Last Thing Dylan Said to Her

Klebold said Dylan only said one word to her as he rushed out the door early the morning of the shooting.

"I hear him bounding down the stairs, past our bedroom door, and really going quickly and heavily out the door, as if he were late," she said. "And I yell, ‘Dyl?,’ ... and he yelled, ‘Bye,’ and then sort of slammed the door.”

Why Sue Klebold Said They Spoke to a Lawyer the Day Columbine Happened

The Klebolds were hit with intense public scrutiny and suspicion after Columbine. One of the things people wondered was why Klebold’s husband called a lawyer when the shooting happened, but she said there was a reason for that.

“It was because Dylan had just gotten off Diversion [a court-mandated counseling program] and we thought, ‘this kid is going to need legal help,'” Klebold said. “My first thought was ... that somehow Eric had done something that turned into something else.

"Your mind doesn’t let you take in a lot, so it may seem absolutely ridiculous, some of the things we were thinking, but that’s what we were thinking,” she added.

Sue Klebold Explains Why She Got Her Hair Done the Day After Columbine

Klebold was severely ridiculed for going to a hair appointment the day after the Columbine massacre. She told Sawyer she had a standing monthly appointment and her next one just happened to fall on that day. She decided to keep it because to her, it seemed like a chance to do something normal amidst the chaos around her.

“I was a wreck. I was a complete basket case. I could barely think. I could barely sit up. I could barely function,” Klebold told Sawyer. “I just didn’t know what to do with myself. I mean, I had nothing to do… I had this hair appointment, and I thought, ‘may as well go because it’ll get me out … of the house.’”

But after the appointment, she said, the hairdresser spoke to the press.

“So it had this very Mary Antoinette feel to it of, like, ‘people are suffering and she's having her hair done,’” Klebold said.

Sue Klebold Didn’t Know Her Son Was Severely Depressed Until After Columbine

When Dylan reached adolescence, Klebold noticed her son, who was once in a gifted program, seemed less interested in doing well in school, that he spent more time alone in his room with the computer he built, and sometimes he seemed moody or irritable, but at the time she thought Dylan was just being a typical teenager.

“Sometimes he would seem, you know, distant or quiet, and I remember asking him, ‘are you OK? Are you sure you’re OK? You seem so tired,’” Klebold said. “And he would stand up and say, ‘I’ve got a lot of homework. I just-- I need to go to bed.’ And I let it go. And that’s the difference. I would dig. If it were me today, I would dig and dig and dig.”

"I had all those illusions that everything was OK because, and more than anything else, because my love with him, for him was so strong," she continued.

Klebold said she didn’t know her son kept journals until after Columbine, and it wasn’t until she began poring over them that she realized Dylan had been journal writing since he was 15 years old about feeling lonely, depressed and suicidal.

“It was just very hard because I wanted to comfort him. I wanted to help him, and it was too late,” Klebold said.

Back then, Klebold said she knew so little about teen depression and thought the problem was with her older son, who was having trouble with drugs. For a time, Klebold said Dylan was the one she didn’t have to worry about. To her, he seemed fine, going to parties and having Friday night bowling with friends.

Several FBI profilers and psychologists analyzed both Dylan Klebold and Eric Harris’s writings and reviewed the so-called “Basement Tapes” -- a series of videos the boys secretly recorded where they discussed their plot to attack the school (the videos were never made public) – and later determined that Dylan likely suffered from severe depression and suicidal thoughts, while Eric was likely a psychopath. Neither boy was formally diagnosed.

Dr. Gregory Fritz, the president of the American Academy of Child and Adolescent Psychiatry, said parents can have a natural impulse to rationalize changes in behavior as just a phase, which can lead to turning a blind eye to the real problem.

“Somewhere between 15 and 20 percent of high school kids say that they have thought about suicide in the past year,” he said, adding that it's OK for worried parents to ask their children if they feel suicidal if there is that concern.

“I’ve interviewed hundreds and hundreds of kids who’ve attempted suicide and they never said, ‘oh my, somebody told, somebody asked me about it and that tipped me over,’” Fritz said. “They’re thinking about it long before anybody asks and often times it’s a relief to have somebody ask.”

Perhaps unexpectedly, Fritz said kids who seem to have the highest expectations for themselves but have trouble coping when things get tough can be particularly vulnerable.

“A small failure … on a grade or a test … can push some of those kids into terrible feelings of worthlessness,” he said.

Sue Klebold Remembers a Confrontation That Became One of Her Biggest Regrets

But about a year and a half before the Columbine massacre, Dylan, then a junior in high school, started getting into trouble. He hacked into the school computer system with friends and was suspended for three days. He scratched an epithet on a locker of another student he thought was taunting him. Then he and Eric Harris were arrested after breaking into a van and stealing electronic equipment. Instead of jail time, Dylan was sentenced to one year of mandatory counseling and community service in a Diversion program.

“And at the time, I thought that was the worst thing I could ever possibly experience,” Klebold said.

After the arrest, Klebold said Dylan acted as if he had done nothing wrong and she gave him one of her lectures about knowing right from wrong.

“I even talked about the Ten Commandments,” Klebold said. “I said, ‘It’s wrong to steal, under-- in no circumstances is this right.’ And then we responded as most parents would. We took away privileges,” which she said included separating Dylan from Eric Harris.

One night, she said she became frustrated at him for not doing chores and thought he needed discipline. She pushed him against the kitchen refrigerator – something she would later say became one of her biggest regrets.

“And I said, you know, ‘you’ve got to stop thinking of yourself. You’ve got to stop being so selfish,’” Klebold said. “I gave him the old Mom lecture. And then I said, ‘and by the way, today’s Mother’s Day and you forgot it.’ And I don’t remember how that confrontation ended. I just remember he softly said, ‘Mom, please don’t push me. I don’t know how much I can control myself.’”

“It wasn’t a scary thing,” she continued. “It was just him nicely [saying], ‘back off, please’ … and then he went out and he got me a gift. It was a little watering can with African violets in it … and I thought everything was fine because he was so sweet.”

At the time, she said Dylan promised her that he was going to turn his life around. Although she was worried about him, she allowed herself to be reassured when he was released early from from the juvenile counseling program with a glowing assessment of his bright future and he went on to be accepted into four colleges. But during this time, Dylan reportedly started to become very close to Eric Harris.

What Sue Klebold Thought of Eric Harris Before Columbine

Several experts who have studied Columbine agree Eric Harris exhibited psychopathic behaviors. He seemed to lack a conscience and empathy, but appeared outwardly charming. He also kept journals and wrote about violence, wanting to have guns, how easy it was for him to lie to people and the pleasure he got from duping others, and included graphic fantasies about getting revenge on people who insulted him.

Dr. Peter Langman, a psychologist and the author of “School Shooters: Understanding High School, College, and Adult Perpetrators,” has studied Columbine evidence extensively and noted that Dylan and Eric were very different.

“Eric liked to draw weapons. He drew swastikas, he wrote about the Nazis,” Langman said. “Now, Dylan drew hearts. Dylan wrote about his search for true love… Eric when he does refer to girls is [in] his fantasies of raping them.”

Sue Klebold said she remembered Eric as being polite, and she had met his parents -- their sons had been friends since 7th grade.

"Of course I met his parents," she said. "We wouldn't have allowed our children to play with anybody that we hadn't met their parents or been in their home. They seemed like very kind and responsible people."

She said another mother warned her that Eric was prone to anger, but Klebold thought it was an overreaction because he had been so polite to her. She didn’t know before Columbine that some frightened parents had alerted the police to Eric's website where he went on savage rants, otherwise she said she wouldn’t have allowed her son to spend time with him.

But she said she does not blame Eric’s parents for their son’s actions and said she talks with them occasionally.

“They are not Eric,” she said. “I don’t feel able to represent them in any way, and I want to make sure I protect their privacy.”

The Harrises did not return ABC News’ request for comment.

What Sue Klebold Thought When Dylan Wanted a Trench Coat

Dylan was a sophomore in high school when his mother said he bought himself a black trench coat, but she didn’t think much of it at the time.

“I was the kind of kid who loved to look different,” Klebold said. “I mean, I was an art major.”

Klebold said she knew the phrase “Trench Coat Mafia,” but to her it seemed like a bunch of students who liked to wear the same style of coat, not an organized group.

Sue Klebold Said She Stopped Checking Dylan’s Room His Senior Year

Once Dylan was accepted to college and about to graduate high school, Klebold decided to respect his privacy and stopped checking his room. But if she had continued, she might have found the hidden sawed-off shotgun and ammunition she later learned was hidden there. Now, she said she would turn his room inside out.

“I would do it as if his very life was depending on it, and I would do it with love,” she said. “In doing something like that we are violating privacy, you are risking damaging the relationship. And of course it’s better to get the conversation... where someone is sharing their thoughts with you rather than having to be sneaky.”

Former FBI agent Dr. Mary Ellen O’Toole, one of the world’s leading profilers of the criminal brain, said parents should feel they have the authority to check their kids’ bedrooms.

“If that room that you pay the mortgage on is being cut off from you so you can’t go in there, you have a problem,” O’Toole said. “Now, does that mean they’re going to go out and commit a mass murder? No necessarily, but you have to know, you have to understand what’s going on with them.”

Sue Klebold Said Dylan Asked Her to Buy Him a Gun

Klebold said one time, when Dylan was a senior, he asked her to buy him a gun.

“I had told him no,” Klebold said. “We kept no firearms in the house.”

A female classmate, the girl Dylan went to senior prom with, legally bought them three guns. The classmate believed the guns would be using for hunting. Before Columbine happened, Klebold had no idea the boys had been training at a shooting range.

The Moment Sue Klebold Stopped Living in Denial about Dylan’s Involvement

After Columbine, Klebold said she refused to believe her son had helped plan the attack and had been a willing participant.

“I believed this was a moment of madness. I believed this was some impulsive fluke that happened suddenly,” she said.

It wasn’t until six months after Columbine, when authorities brought her and her husband in to go over the evidence, that she learned the truth, that Dylan and Eric had spent months forming a plan to attack their school and then had carried it out. Detectives showed them a timeline of the day, and in her book, Klebold made herself examine every shot her son fired and the people he killed.

“I try to be as honest about that as I could. I didn't want to make it graphic, but I wanted to make it honest,” Klebold said. “Because, you know, from a mother's perspective, of course, there is a tendency to want to soften all the horrible things that he did.”

It was also during that meeting that detectives showed them more than three hours of videotapes Dylan and Eric had secretly recorded in their bedrooms, which became known as the “Basement Tapes” because Eric’s bedroom was in his family's basement.

“It was horrible to see those tapes,” Klebold said. “They were posturing. They were acting tough. They were talking about all the horrible things they were planning to do. It was vile.”

“I remember at one point standing up, because I thought I was going to be ill, and I might have to run out of the room,” she added.

The tapes were never released to the public and they have since been destroyed.

Sue Klebold Believes She Could Have Stopped Dylan If She Had Recognized Warning Signs

Two months before the Columbine shooting, the Klebolds met with Dylan’s English teacher to discuss a story Dylan had written about a tall man in a black overcoat who fills a duffle bag with weapons and guns down a group of “college preps.”

She said she and her husband asked Dylan about the paper twice but when she said he told her he didn't have it, they let it go.

“I did not grasp the seriousness of that paper,” she added. “I don’t think any of us did at the time.”

The counselor told ABC News that in a pre-Columbine world, he didn’t see the paper as a threat at the time.

Mary Ellen O’Toole said that one thing, such as a violent writing for a class, might not be enough to take action, but it could be a piece of a larger puzzle.

“It's not a red flag that would be indicative of someone-- absolutely going out and becoming violent,” she said. “But it's enough of a red flag to say, 'OK, let's get going. Let's take a look at this young person.'”

Looking back, Klebold believes if she had realized something was wrong with her son, she could have prevented him from carrying out the Columbine massacre.

“I don’t ever for a moment mean to imply that I’m not conscious of the fact that he was a killer, because I am,” she said. “[But] if I had recognized that Dylan was experiencing some real mental distress... he would not have been there. He would have gotten help.”

After Columbine: Experts Offer Tips for How to Talk to a Troubled Child

Resources for Suicide Prevention, Mental Illness Concerns, Helping Parents Cope

FULL COVERAGE: Diane Sawyer's Exclusive with Sue Klebold