In the spring of 2003, things were looking up for 17-year-old Christine Paolilla, a shy teenager who had always struggled to fit in. She was friends with two popular girls at school -- Rachael Koloroutis and Tiffany Rowell -- and she had been voted "Miss Irresistible" by the student body at Clear Lake High School in suburban Houston.
But on the afternoon of July 18, 2003, Christine's life changed forever. Four youths were found shot multiple times at point blank range in a home in the placid Clear Lake neighborhood. Two of the victims were her friends, Koloroutis and Rowell, who had recently graduated. Koloroutis was planning to attend college in the fall.
The police were stunned, and stumped as to who would want the youths dead.
"There was a lot of rage and anger behind these killings and so, therefore, we thought ... perhaps there was a personal relationship between the victims in the home and the killer," Houston Police Sgt. Brian Harris told ABC News.
For the full story watch "20/20" tonight at 10 p.m. ET
It would take three years to unravel the mystery. After numerous dead ends, police caught a big break in July 2006, when an anonymous tipster called. The tipster described facts only the killers could have known and gave them two names -- a boy, Chris, and a girl: Christine Paolilla.
Christine was arrested, tried and convicted of the crime. She is now appealing the conviction. "Chris" was Chris Snider, Christine's boyfriend in high school, who committed suicide after he learned police were on his trail.
The question that haunted everyone, including the police, was, "Why?" The details didn't seem to add up. Rachael and Tiffany had befriended Christine and offered her advice, including beauty tips. The friends even carried pictures of each other in their wallets.
Why would Christine want to kill her friends?
Christine's parents, Lori Paolilla and stepfather Tom Dick, spoke to "20/20" exclusively about their daughter's life before July 18, 2003.
Christine Paolilla spent her childhood in suburban Long Island, N.Y. Her mother, Lori Paolilla, told ABC News that the girl was "very outgoing, outspoken, though shy at times... she was the apple of Daddy's eye." Christine's father, Charles Paolilla, was a construction worker and her mother stayed home to raise Christine and her older brother, John.
Christine was just 2 years old when tragedy hit. Lori Paolilla described the painful day.
"Her father got up and went to work and never came home," she said.
Charles Paolilla was killed by falling bricks during construction on a high-rise in New York City.
"I had to go home," Lori Paolilla told ABC News, "and tell my children that Daddy won't be coming home anymore."
A few months later, Christine's grandfather and great-grandmother also passed away. According to her mother, Christine started asking questions: "'Mommy, I don't understand ... why is it that people I love go away?'"
Her mother says she tried to raise the children on her own, but the pain of losing her husband led her to drug addiction. She eventually lost control of her world and, temporarily, custody of her children. At age 7, Christine went to live with her grandparents.