When asked if there were other factors that contributed to the division in their marriage, Reiser replied, "Well, she slept with my best friend."
Hans believes that his childhood friend Sturgeon introduced his wife to drugs and exotic sex in a twisted act of revenge.
"He offered to have a relationship with me and I said no, cause I'm just not gay," said Reiser. "He was jealous, and then later he saw me and Nina having this family, and I think he saw it as something that he couldn't have. … When people feel rejected they do crazy things."
Doren sees the relationship between Sturgeon and Nina differently.
"When Hans went to Russia, he presented Sean to Nina saying, 'This is my best friend. He's here to take care of you. Whatever you need, go ask Sean,'" said Doren, adding that she never worried that Sturgeon was involved in Nina's disappearance. "He loved her dearly," she said.
Sturgeon won't talk publicly until after the trial, but after Nina's disappearance he made a bold confession to police, so outrageous that the judge won't allow it to be mentioned in court. He claims to be a serial killer, but said he is not responsible for Nina's death.
The morning of her disappearance, Nina took her two children grocery shopping and then headed to her mother-in-law's house, where Reiser was to take the kids for the rest of the Labor Day weekend. Nina had plans to spend the evening with her best friend.
"She was supposed to be at my house at 6:30, and I started to get worried at 6:35," said Doren. "'Cause she was never late, and then not to call was very, very unusual."
Doren called Nina's cell phone every half an hour through the night, and by Monday morning she was panicked. Doren said that on Tuesday, before anyone knew for sure that Nina was missing, Reiser made a rare appearance at the children's school and told the school to get in touch with him in case of trouble.
"That was very unusual for him, to go to the school," said Doren. "And especially to leave his cell phone number as an emergency contact. Nina normally took care of that."
When the police attempted to question him, Reiser tried to evade them, once even jumping out of his car and running for blocks. His attorneys didn't want him talking about the evidence against him before trial, but his father said he may be to blame for some of his son's strange behavior. He told Reiser it might not be the police who were tailing him at all, but perhaps someone more sinister connected with Nina.
"I said, 'If people are following you, either they identify themselves who they are legitimately or you don't let them follow you,'" said Ramone Reiser.
"20/20" traveled to St. Petersburg, Russia, to meet Nina's family. Her mother, Irina Sharanova, said her daughter just fell in love with the wrong man.
"[Hans] often screamed at her," she said. "If she spoke on the phone, if she was walking with children, if she was talking with us on the phone. He was always angry with her."
When asked if Nina was fearful, Sharanova said, "Yes, that's right."
Before Nina's disappearance, she accused Reiser of pushing her down and was granted a restraining order against him.
"I told her to be afraid of him," said Doren.