June 30, 2009 -- Nathan Gann, a clean-cut honors student who attended the University of Arizona, was a self-described computer geek. During the summer of 2007, 19-year-old Gann was busy blogging and posting videos of himself, which were goofy but showed a sense of promise and ambition, all over YouTube.
"I love the stars, ever since I was a child," he wrote on his MySpace page. "Because of them I have always wanted to reach for the stars. My dream goal is to be an astronaut."
Nathan's younger sister, 17-year old Brae Hansen, was equally driven. A high school honors student, who had recently returned from a church trip performing community service in Puerto Rico, she embraced life.
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But in July 2007, disaster struck. At first it looked like a home robbery gone wrong: Brae Hansen told the 911 operator that a masked intruder had shot her 63-year-old stepfather, Tim MacNeil, in their San Diego home.
But as police investigated the July 2007 shooting death, two unlikely suspects emerged -- MacNeil's own stepchildren.
Why would these teens want their stepfather dead? The evidence didn't seem to add up. Brae reportedly adored her stepfather, whom relatives said she always affectionately called "Daddy."
Torturous Upbringing Haunts Kids
Tim MacNeil had met and married their mother, Doreen Hansen, a schoolteacher, when Nathan and Brae were 7 and 5 years old. Erin MacNeil, Tim MacNeil's daughter from a previous marriage, told ABC News' Mary Fulginiti that Brae and Nathan were part of the family.
"They were his kids," she said. "He treated them just like any other ... just like he treated me."
However, Brae and Nathan were two kids who had spent their lives desperately trying to earn their mother's love.
"She was physically violent with him, hitting him ... with sticks," claimed Nathan Gann's attorney, Ricardo Garcia. She would, ridicule him. He would be in the bathroom ... and [she would] bring his sister in and point out his penis and ridicule him about it in front of his sister."
Suicidal Thoughts Torment Mother
According to Nathan, his mother's verbal threats were just as violent as the physical ridicule.
"She had threatened to kill me several times," he told ABC News' Mary Fuginiti in an April 2009 interview. "She had told me that she was gonna kill me and bury me in Mexico."
Brae reveals that she, too, was tormented by their mother's suicidal thoughts. "She would just say, you know, 'What would you do if I ... you know, drove off the Coronado Bridge? What would you do if, you know, I just burnt the house down?' And this is when I'm, you know, 10, 7."
In a childhood of chaos, Tim MacNeil appeared to provide an oasis of sanity for the kids.
"He'd say, 'Doreen, that's enough'," Nathan told ABC News. And according to Brae, MacNeil was the one person in the household she believed she could trust.
When Nathan Gann was 12, MacNeil arranged for him to go live with his grandmother in Arizona to escape the turmoil at home. But after he left, Nathan's name was rarely mentioned, according to MacNeil's daughter, Erin.
"It was really weird. ... It was never spoken about to me. He was just gone," she said.
Away from his mother, Nathan blossomed and became an honors student. Brae, who remained with her mother and stepfather, continued to thrive in school, becoming interested in student government, despite obstacles at home.
According to her lawyer, Troy Britt, Brae's dream was to go to law school someday, following in her stepfather's footsteps.
"That's what her dad, Timothy MacNeil, did. He went to law school. He was an attorney. And someday, she wanted to be a judge," Britt told ABC News.
But as their mother's condition worsened, she continued to take it out on her daughter, straining their relationship and pushing Brae to the brink.
"It got to the point where I had to move to Tucson, because I thought she was going to kill me," said Brae Hansen.
To escape her mother, Brae moved in with her brother, now a college student in Arizona. Six months later, a terrible tragedy took place.
Siblings Become Prime Suspects
"My mom called and threatened suicide. ... She said that she was gonna die. She was like, 'I need to talk to Brae,' " Nathan recalled of the night in June 2006.
It wasn't the first time his mother had threatened suicide, and Nathan said he perceived it as another empty threat.
"I didn't know that she had already taken a whole handful of pills," Nathan said.
The next morning, MacNeil found his wife dead in the office of their home.
"I guess she just wanted to say goodbye" said Hansen.
For two kids who sought their mother's love, her death was a devastating blow.
"I was bawling. I couldn't stop crying for days," Nathan said.
Despite their tumultuous and tragic relationship with their mother, the siblings seemed to be coping well, and a feeling of normalcy was restored. Nathan continued school at the University of Arizona, where he started his own computer company, and Brae moved back home with her stepfather, where she became used to having MacNeil all to herself.
But when her stepfather began dating and fell in love with another woman named Kim, Brae questioned the role of a new woman in her stepfather's life.
"He was basically going to cut me out of his life completely," she said in the videotaped police statement. "I knew he'd already chosen his girlfriend over me, and it hurt really, really bad because this was the man I thought loved me, and was my dad."
In July 2007, MacNeil was killed by four gunshots, and both stepchildren became suspects his murder.
But family, friends and investigators were all asking the same questions: What possibly could have caused the siblings to kill their seemingly beloved stepfather, who always stood up for them?