On March 7, Bush walked into her school cafeteria with her father's gun and started shooting. "There was a deep part of me that just exploded," Bush tells Connie Chung in a 20/20 interview, "I'm not normally like that."

Her only victim was 13-year-old Kimberly Marchese, who was shot in the shoulder. Though it will be a long time before Marchese has full use of her arm again, she says it didn't hurt when her classmate shot her. "It was like a dead person's arm attached to me," she remembers.

What would push Bush, a seemingly shy girl, to the brink? For years she left a trail of clues but her parents, teachers, counselors and friends did not pick up on the warning signs.

Bush grew up in a loving family and had no history of violence. The serious, introverted, deeply religious girl was, in fact, unusually caring. "We used to tell each other how lucky we were to have a child with such a good heart," says Bush's father.

Excruciating Depression

Bush was clearly different from the other kids. She was a nonconformist and by the time she entered junior high school, they began to make fun of her. She became depressed and frequently skipped school without her parents' knowledge. As her depression worsened, she began to cut herself with a razor blade and often contemplated suicide.

Increasingly concerned, her parents transferred Bush to Bishop Neuman High School, a small parochial school. It was Bush's chance for a fresh start.

School records indicate Bush was "depressed," "withdrawn," and "very defensive if someone gets too close." Teachers said she was "paranoid and felt persecuted." And the teasing continued. Marchese, who was one of Bush's few friends, says, "They would say that she was like a freak or she was weird or she was like messed up."

In Williamsport, Pa, shortly after Bush's sentencing, ABCNEWS' Chung spoke with her about what led to her violent act and how she views the event now, more than a month later.

Self-Mutilation

Elizabeth Bush: [At the old school] they'd just call me an idiot, stupid, fat, ugly, whatever … One incident was I was walking home from school and five or six kids were behind me and they started throwing stones at me … They were just kind of laughing and I don't know why they were doing this but they were barking at me. I don't know.
Connie Chung: So what did you do?
E.B.: I kept walking and started to cry. I kept walking faster and faster till I got home and then I just stayed in the house.
C.C.: How serious do you think your depression was?
E.B.: Really bad … I started self mutilation … I cut myself … I was angry at myself for being different.
C.C.:Why would you hurt yourself more?
E.B.: Well, people express their anger different ways. Crying helps, that didn't help me. So I thought maybe I'd try this and maybe it will help. The pain just takes away all your depression and for a minute you're not depressed anymore.

Struggle With Friendship

The classmate Bush would later shoot was one of the first to extend her friendship in her new school. Marchese was the cheerleading captain, a basketball and soccer player and even though they were polar opposites, Bush considered her a friend and revealed deepest secrets to her.

E.B.: I confided in her because I felt I could trust her … she saw the marks on my arm. She was like, 'ugh, what did you do, you know?' And I said, 'Never mind.' And she kept asking me, hassling me about it. And I said, 'OK, I cut myself.' And she was kind of like, 'why?' And I said 'because I get depressed, that's all, end of story.'
C.C.: Do you think she told other kids?
E.B.: Oh yeah … she acted as if she was my friend to my face. But when I left or something, she'd turn around and talk about me.
C.C.: Negatively?
E.B.:Yes. I know this because she talked to a friend of mine about, about me. She was laughing. She was calling me a freak and all this stuff.
C.C.: When you heard that, what were you feeling inside?
E.B.: I was very hurt that she'd do that to me … those feelings, those thoughts that I told her, they were never supposed to be revealed to anybody; and that's what she did.

Considering Suicide

E.B.: I just wanted to die. Because I had so much hell on my shoulders … if what people are considering fun is to tease me, then I'll just stop living and they won't be able to hurt me anymore … at first I was thinking of shooting myself because I wanted to show her this is what you made me do.
C.C.: You wanted to show Kim Marchese?
E.B.: Yes. I wanted her to see it. I wanted her to see everything.
C.G.: To see you committing suicide?
E.B.: Yes … I wanted her to know my pain.

'I Was Hysterical'

C.C.: Did you want to harm Kimberly Marchese?
E.B.: No, I didn't.
C.C.: Did you point the gun at her?
E.B.: Yes, I did.
C.C.: Did you pull the trigger as it was pointing at her?
E.B.: Yes I did. Yes I did but I was thinking maybe if I aimed far away from her — I didn't think I would hit her. I didn't think I hit her with the gun because she ran.
C.C.: What happened when you put the gun down?
E.B.: The assistant principal — I went into her arms and I was bawling and I was like 'Oh, my God, I shot someone — I tried to shoot somebody. Oh my God.' I was screaming and crying. I was hysterical.

'I Chose to Do It'

Within weeks, Bush appeared in juvenile court to plead guilty to a host of criminal charges, including attempted homicide. The victim's father was influential in keeping her case out of adult court. Sobbing, Bush apologized to Marchese. Bush was sentenced to a juvenile facility and could remain there until she is 21.

C.C.: Did any violence on television or violence in music contribute to what happened to you?
E.B.: No.
C.C.: Did the school shootings that occurred recently contribute to what happened to you?
E.B.: No … I had a choice to do this or not. I chose to do it.
C.C.: Do you think this would not have happened if there not been a gun in your home?
E.B.: Probably not. But I'm not blaming that.
C.C.: You've had a lot of time to think about this incident … What goes through your mind now?
E.B.: It plays over, over and over all day and all night ... And it hurts me because I hurt somebody else.
C.C.: Why did you do this interview?
E.B.: Why did I do this? Because I want to stop as many school students as I can. I want to show people that there are other ways. I want to reach out to kids that are just like me they're depressed. They want to hurt themselves, they want to hurt others. I want to stop all of that.