Four months after a Colorado man strangled his pregnant wife and smothered his two young daughters to death, the woman’s grieving parents are still trying to come to terms with the horrific murder, they said in their first interview.

The New York Post reports that they thought their daughter was in a happy marriage.

“That’s the big word — Why?” Frank Rzucek, father of Shanann Watts, 34, told ABC’s 20/20. “He must have snapped, because there’s nothing else I could figure out what happened to him.”

“We did scream at God, yes we did,” Sandra Rzucek, Shanann Watts’ mother, added on the program. “The devil comes to you in any shape, size and form and can be transformed into anything.”

Christopher Watts, 33, pleaded guilty to the heinous slayings of Shanann — who was pregnant with a son the couple had already named Nico — and their two daughters, four-year-old Bella and three-year-old Celeste, in exchange for prosecutors sparing him the death penalty.

Watts initially played the role of concerned husband and father, telling police and news outlets that he had no idea what happened to his family.

But when, two days after they were reported missing, the girls’ lifeless bodies were found stuffed into oil tanks and their mum’s remains were uncovered in a nearby shallow grave, Watts changed his tune and admitted strangling his wife, but only because, he claimed, he saw her abusing Celeste.

Investigators quickly determined that that story too was “a flat-out lie” and that Watts wanted a fresh start with his mistress.

The Rzuceks called Shanann a “phenomenal” mother who would “do anything for her children” and battled lupus for a long time.

The family at first “loved” Watts, whom Shanann had met through Facebook, they said on the program.

“You just don’t sleep very good with lupus,” Sandra Rzcuek said. “She had her head on his lap, and they were watching TV. He didn’t move. He didn’t get up to go to the bathroom. He didn’t get a drink. He let her sleep for four hours. I told her, ‘God must’ve sent him to you.’”

It wasn’t until Shanann went with the girls to visit her family in North Carolina for six weeks — while Chris stayed behind — that any marriage problems started cropping up.

Once Shannan and the kids arrived home, Shanann’s friend Addy told 20/20 that Shannann had texted her to say that Watts “had changed” and told her he didn’t “want another baby.”

Watts was sentenced in November to five life sentences, three to be served consecutively, without parole.