Sept. 23, 2009 -- It was a horrific crime, even by the big-city standards of Houston. On the afternoon of July 18, 2003, two people entered a home in the manicured suburb of Clear Lake and shot to death the four young people inside. There were no obvious suspects. No arrests were made.
Police worked the case for three years. They interviewed dozens of the victims' friends and acquaintances, many of whom were classmates at Clear Lake High School. They fielded hundreds of tips, none of which panned out.
Family members of the victims pitched in. Leading the effort was George Koloroutis, father of victim Rachael Koloroutis, who was 18 when she died. He printed fliers, sent out mass mailings and went door to door to raise more than $100,000 for a reward in the case. Koloroutis also arranged for composite sketches of the potential suspects to be released to the public and posted on more than a dozen billboards along Houston freeways.
"He was able to think with his head when all I wanted to do was think with my heart. But George never gave up," said Ann Koloroutis, Rachael's mom, in an exclusive interview with "20/20."
"I thought that would have a really meaningful and loud impact on the community," said George Koloroutis of his efforts. "And it would ... make these killers come out from hiding."
Then, in July 2006, a tipster called a police hotline with details only an insider could have known. He knew the position that Rachael, a recent high school graduate, had died in. She had been crawling on the floor. A phone lay near her hand. She was trying to dial 911.
"This is your shot," Houston Police Sgt. Brian Harris told ABC News, recalling the moment. "This is our one shot to make this happen."
The tipster mentioned two perpetrators. One was shrouded in mystery; the tipster thought his name was Chris.
There was no question about the other perpetrator. She had seemingly told the anonymous tipster everything about the crime. She had been a classmate of Koloroutis and another victim, Tiffany Rowell, at Clear Lake High School. Her name was Christine Paolilla.
For the full story, watch "20/20" Friday at 10 p.m. ET
The police hunt, arrest and trial that followed shocked the victims' families, the high school and the greater Clear Lake community. Christine, Rachael and Tiffany, it turned out, were more than classmates. For a while they had been close friends. Also killed were Tiffany's boyfriend, Marcus Precella, and his cousin, Adelbert Sanchez. The looming question was what, exactly, had gone wrong -- and why it all had ended in such violence.
Life at Clear Lake High
Clear Lake High School attracts kids of oil executives and NASA engineers. And in the inevitable social hierarchy that exists in any high school, Rachael Koloroutis and Tiffany Rowell stood on top.
Tiffany was a talented actress who dreamed of becoming a social worker. Rachael was into art and creative writing. Like Tiffany, she was blessed with the two things every teen craves: good looks and popularity.
"When you first saw Rachael Koloroutis, she just struck you as this beautiful girl," said Jennifer Grassman, a friend. "I mean, she could have been a model."
It was not the same for Christine Paolilla, a shy outsider who'd always struggled to fit in. Christine had tragically lost her father, a construction worker killed on the job, when she was just 2 years old. Her mother, Lori Paolilla, had trouble with drugs. As a young girl, Christine was diagnosed with a hair-loss condition called alopecia. She suffered ridicule at the hands of her classmates.
But thanks to Rachael and Tiffany, things for Christine finally took a turn for the better. Though they were a year ahead of Christine, they befriended her. And with their help, she transformed herself from an awkward misfit into a high school Cinderella.
"She was voted by the school, 2003, Miss Irresistible at Clear Lake High School," Lori Paolilla told "20/20." "They did it because they felt that she was the person who they just loved, because of the way she was, the person she was."
'Like an Earthquake'
Rachael and Tiffany graduated in May 2003. On the afternoon of July 18, the two girls were doing what teens typically do, hanging out at Tiffany's house, unsupervised, watching TV with Tiffany's boyfriend Marcus and his cousin, Adelbert. Then they heard a knock at the door.
"Once they opened the door, they were probably attacked within minutes," said Harris. "And they were met with absolute fury."
What had been a lazy summer afternoon became the scene of a suburban massacre. Bodies sprawled across furniture.
"The kids themselves are drenched in blood," said Harris. "There's blood spatter that's on the walls. There's bullet holes from rounds that have gone into the victims. ... We had four recently graduated high school kids that were slaughtered. I can't even say just murdered, but literally slaughtered."
Harris says Houston police were as perplexed as they were horrified.
"You know, this was not a group of kids that would be considered thugs or gangsters by any means," he said. "This was an area that could be anybody's neighbors' kids."
The victims' families were equally stunned. "It was like an earthquake to our family. And it was devastating," said George Koloroutis.
Ann Koloroutis said a pervasive fear set in. "People were angry and afraid," she said. "Nobody knew who did this. It was young kids in the middle of the afternoon in a nice neighborhood."
In the three years between the murders and the tipster's call to police, Christine Paolilla walked with her class at Clear Lake High, but she'd begun having problems with drug abuse. She spent time at a rehab clinic and married a man, Justin Rott, she met there.
But another dramatic turn in her life was shaped by events long past.
When Christine turned 18, she gained access to a trust fund established for her after her father's workplace death. It totalled $360,000.
"She knew that she probably didn't have to work at a McDonald's," said Lori Paolilla, "which she never, ever wanted to do."
Instead, Christine eloped with Rott. In an interview with ABC, Rott said that a honeymoon period for the couple was followed by despair. His new wife, he said, was haunted by memories of her murdered friends.
"She would see Rachael's face," said Rott. "I mean she'd literally start hallucinating. She would wake up in bad dreams, uh, see certain things on TV and movies, she'd start crying, panic attacks."
As she fell apart emotionally, Christine and Justin both turned back to drugs -- specifically, heroin. They spent months draining her inheritance.
"A $500 withdrawal ... every day," said Rott.
The pair holed up in a hotel room in San Antonio, shooting drugs day after day, month after month.
"She never left that room one time," said Rott. "The whole, over-nine-month time, the only time I ever left was to get food or drugs."
Her parents thought their daughter might be lost for good.
"We didn't know if she was alive," Tom Dick, Christine's stepfather, told ABC.
She stayed in the room until the police came for her.
"It almost looked like a murder scene inside this hotel room," said Harris. "There's blood on the wall, hundreds of needles, used needles on the ground, boxes of brand new needles ready to go and then, literally, about 80, 85 needles lined up on a dresser with heroin inside 'em. It reeked."
Police Close In
After the July 2006 call from the tipster, the police investigation advanced on two fronts. Using bank records, detectives were able to track Christine to her San Antonio drug den, where they arrested her.
Then they began to unravel the mystery of the second perpetrator, "Chris."
In video of a police interrogation of Christine, her fear of "Chris" is palpable.
"You're sure he's not going to be able to get me in here right?" she asked the detective questioning her. "Promise me that."
It turned out that "Chris" was Chris Snider, whom Christine had dated in the year leading up to the murder. Snider was a loner, about two years older than Christine, who had done jail time for armed robbery. According to Christine's parents, he was very controlling and he resented her female friends, including Rachael and Tiffany.
During her police interrogation, Christine said she was eventually able to get Snider out of her life and she had no idea where he had gone. The police, however, were already on his trail.
The tipster, whose identity was never disclosed, had told police the murder weapons belonged to Snider's father. Police executed a search warrant on Snider's parents' home in Kentucky.
"Lo and behold, in the father's bedroom was a little gun safe," said Harris. "And in that gun safe were the murder weapons." Detectives then used Snider's MySpace page and phone records to track him to Greenville, S.C. But Snider learned about the manhunt before they arrived.
The police found his body in the woods.
"He committed suicide," said Harris. "Overdose. Which is kind of ironic, for this big tough guy who brutally slaughters -- these four high school kids. He went out the coward's way."
Christine, now 20, blamed Snider for the crimes. The state responded by charging her with first-degree murder.
Rachael Koloroutis' family was shocked anew when they learned that one of her high school friends had been arrested and charged in the crime.
"I just remember sitting in my car and just shaking," said Tiffany Koloroutis, the elder of Rachael's two sisters.
After all, why would Christine want to kill her own friends?
'A Lot of Rage and Anger'
Police had long suspected that drugs had played a role in the quadruple murder. They had discovered early on that despite its tame exterior, Tiffany's house had become a teenage hotspot.
"We learned that there was drugs that were sold from that residence and that these kids were involved in drugs," said Harris.
There had been no adults in the house for a while. Tiffany's father had moved away and let her stay there alone to finish her senior year of high school. Police believe Tiffany's boyfriend, Precella, was selling party drugs like ecstasy and marijuana to Clear Lake students.
Now Christine told police that she and Snider went to the house on the day of the murder because Snider wanted to score drugs. But that didn't explain everything, for the detectives.
"Ultimately, it never made sense," said Harris. "I mean, you look at the brutality of the crime. There was a lot of rage and anger behind these killings."
In her police interrogation, a distraught, chain-smoking Christine Paolilla said that when she and Snider parked near the house, it became clear Snider was up to no good.
"I was right behind Chris," she told an interrogator. "I stayed behind him, like, the whole time because I felt so bad, I was just so scared."
The detective asked what she felt.
"He was gonna, you know, shoot me," she replied. Once the two got inside, she said, Snider became aggressive.
"Chris started arguing with Marcus and it was getting loud," she said. "And that's when I heard the first gunshot. I wanted to run but I couldn't, I felt so scared and I felt so sick. ... And then it felt like I kept hearing like the bubble-wrap noises, like pop, pop, pop, pop, pop, pop."
Christine said Snider had forced her to carry a gun. Then, she said, he made her fire it.
"So the gun was in your hand ..." an interrogator asked.
"He was holding on to it, too," said Christine. "And I was scared, and I was crying, and like, uh, I had made the gun go off, not purposely ... a million times."
Harris summed up her defense.
"In essence, she claims Chris Snider killed all the people and she just held the gun," he said.
Christine said Snider threatened to kill her and her family if she talked, so she composed herself and reported for a work shift at Walgreens.
"I was so scared and ... at the time I felt that if I didn't do what [Snider] was going to say, he was going to shoot me," she said.
"There's been times where he'd get almost satanic talking about people, saying, 'I wonder what I'd be like to kill someone.'"
Christine Paolilla began her trial on murder charges with the defense that her boyfriend at the time, now dead, was responsible. A Texas jury would decide her fate.
For the story of Christine Paolilla's trial on murder charges, visit the "20/20" Web site Thursday. For the full story, watch "20/20" Friday at 10 p.m. ET