The police investigation soon revealed what appeared to be cracks in the couple's seemingly perfect marriage. When detectives searched Peterson's computer, they discovered pornographic images of men, including e-mails from Peterson soliciting sex from a male escort. The prosecution suggested that Kathleen discovered his homosexual affairs and confronted Peterson the night she was murdered.
"According to the persons that know [Kathleen] well … she would have been infuriated at learning that her husband, who she truly loved, was bisexual and having an extramarital relationship not with another woman but even with a man, which would be humiliating and embarrassing to her, said Freda Black, then the assistant district attorney. "We believe that once she learned this information, that an argument ensued and a homicide occurred."
Peterson's five-month murder trial captured the headlines with sensational stories about Peterson's secret double life. But behind the scenes, his own family split over his guilt or innocence.
After reviewing the autopsy report, Kathleen Peterson's daughter, Caitlin, became convinced her mother had not slipped on the stairs. "My conclusion, which I stand by 100 percent, was that Michael murdered my mother," Caitlin told "Primetime."
Caitlin's public denouncements of Peterson created a deep rift with stepsisters Margaret and Martha, who steadfastly supported Peterson. .
"I did consider Martha, Margaret my sisters and, you know, my mother considered them her daughters. But my goal, my strength in life is completely opposite," said Caitlin. "It's in seeking justice for my mother; that's what I'm here for."
But just as the trial was about to begin, there was another startling revelation.
Nearly two decades before Michael Peterson found his wife at the bottom of a staircase, a close friend of his in Germany, Elizabeth Ratliff, had also died after a fall down a staircase. Peterson was reportedly the last person to see her alive.
Authorities in Germany had determined that Ratliff's death was the result of a fatal stroke that sent her toppling down the stairs. And no questions were raised when her close friend Michael Peterson adopted her daughters, Margaret and Martha.
But with Kathleen Peterson's death at the bottom of the stairs, authorities began to wonder: Was this a tragic coincidence or a pattern? To find out, the prosecution exhumed Ratliffe's body. A new autopsy concluded that Ratliff had also been the victim of a homicidal assault.
"The DA is trying to say basically that our dad killed our birth mother and our mother," said Margaret Ratliffe. "But where are we sitting? We're sitting behind our dad."
"I think Liz died a perfectly natural death," said Michael Peterson. " I don't think anybody did it. But if somebody did do it, well it sure as hell wasn't me."
But Caitlin, Kathleen's daughter, told ABC News, "When I found out the evidence of Elizabeth Ratliff's death and that it was very similar to my mother's death, there was a possibility that I had been living with a man for 13 years who had possibly committed murder 18 years ago and all this time, he was, there was this big secret, something that I didn't know about him and that's very scary."
On Oct. 10, 2003, the jury found Peterson guilty of first-degree murder. He was sentenced to life in prison without the benefit of parole.