Sandra and Frank Rzucek said that their daughter and grandchildren's murders were "evil at work."
Chris Watts seemed increasingly distant in the weeks leading up to his family's murders.
In an interview with ABC's "20/20," Sandra and Frank Rzucek, Shan'ann Watts' parents, called the murders of their daughter and grandchildren "evil at work." Though they believed Shan'ann was in a happy marriage, they said she told them that something had changed in their relationship just before the killings.
"She just couldn't understand why he was so cold," Sandra recalled. "... Just as cold as ice and not responding and replying."
Sandra counseled her daughter to give her husband "space."
"Little did we know," said Sandra.
The couple last saw their pregnant daughter over the summer, when she spent six weeks visiting them in North Carolina with her daughters, 4-year-old Bella and 3-year-old Celeste.
While Shan'ann was away, Watts had affairs with multiple women, and when she returned, things got worse.
Texts exchanged between Shan'ann and a friend just days before her death show how fraught their relationship had become. "Chris told me last night he's scared to death about this third baby," wrote Shan'ann in one message. "And he's happy with just Bella and Celeste and doesn't want another baby."
Inside Edition spoke to "20/20" co-anchor Amy Robach, who interviewed the Rzuceks about their late daughter.
"It looked like the perfect family and Shan'ann's parents tell me it was the perfect family," Robach said. "It was just something in the last two months that made him snap.
"Was it an affair? Something else? Shan'ann's parents say it was 'evil at work,' there's no other way to explain it."
Added Robach, "What makes this story so is fascinating is that Chris Watts had no warning signs. No one ... thought him capable of any kind of violence."
Watts was sentenced to five life sentences for the murders of his pregnant wife and children last month after pleading guilty to avoid the possibility of the death penalty.
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