Leading a double life: While appearing as a family man, Chris Watts wrote love letters to another woman before murdering wife, 2 daughters

It was Aug. 14, more than 24 hours after Shanann Watts and her two young daughters, Bella and Celeste, had been reported missing from their home in Frederick, Colorado, when the community’s idyllic perception of the Watts household was shattered forever.

The news of their disappearance quickly captured headlines, sending shockwaves through the town.

The only hint of trouble at home was a comment from her husband, Chris Watts, to a reporter from ABC's Denver affiliate KMGH-TV the day after his family had gone missing: “We had an emotional conversation. That’s all I’m going to say.”

But what was happening behind closed doors in the weeks before the pregnant mother and her young children went missing?

From the outside, Shanann Watts seemed ecstatic about the summer she was going to spend with her family. She shared on Facebook that she, along with her two girls, would be visiting relatives for an extended period in North Carolina.

“We had a lot of fun,” Shanann Watts’ father, Frank Rzucek, told “20/20” in an exclusive interview, remembering fondly visiting the beach with his granddaughters.

“We played in puddles. We went to the zoo,” his wife and Shanann’s mother, Sandra Rzucek told “20/20” in an exclusive interview.

Chris Watts had stayed at home in Colorado working – but even from afar, Shanann Watts began to sense a change in her husband.

Sandra Rzucek said her daughter had told her that the transformation happened over the course of “a few weeks, maybe a month.”

Nickole Atkinson said her friend Shanann Watts “didn’t know what was wrong.”

“She’d just told me that Chris said he wasn’t happy anymore,” Atkinson said.

Court documents released last week that included text messages between Chris and Shanann Watts show that she felt their six-year marriage was unraveling.

On July 10, she asked him: "You ok? lt's like you don't want to talk I kept trying to talk and I had to dig it out of you?"

Chris replied, "I'm fine baby": "I didn't mean to seem short Boo. I love you to the moon and back..."

Two weeks later, Shanann expressed her frustration again in texts to Chris: "While you are working and living the bachelor life I'm carrying our 3rd and fighting with our two kids daily... It's not hard texting love you and miss you."

“If you are done, don't love me, don't want to work this out, not happy anymore and only staying because of kids, I NEED you to tell me," Shanann Watts wrote in one text.

Chris Watts responded with a chilling assurance: "I'm not just staying because of the kids. They are my light and that will not change. I'm not sure what's in my head.”

“She was missing him a lot. She was looking forward to him getting there,” said her friend Addy, who asked that her last name not be used.

Chris Watts joined his family during their last week in North Carolina. But the couple still didn’t get along.

In texts with Addy, Shanann Watts revealed that her husband had told her he was “scared” about having a new baby and he was happy with his two daughters.

“He has changed,” Shanann Watts told her. “I don’t know who he is.”

“Everything will be fine once the baby comes,” Addy replied.

Shanann Watts considered that he might be having an affair, but told another friend she didn’t think he had “the game” to pull it off.

With her crumbling marriage heavy on her mind, Shanann Watts went on a business trip to Scottsdale, Arizona.

“She wasn't herself,” Addy said. “She was trying really hard, but her mind and her heart were really with what was happening in her marriage.”

Atkinson, another friend, said she was “trying to figure out how she could fix it. … She was very much analyzing everything she said or did.”

According to court documents, it was an alert to a credit card charge of around $60 for a salmon dinner from a sports bar that made Shanann Watts suspicious that her husband could have been having an affair.

It turned out he was with his coworker Nichol Kessinger.

Kessinger later told police she and Chris Watts “got along really well.”

“It was very comfortable for me. I enjoyed it. I think he did very much as well,” she told police.

Kessinger said he’d told her that he was separated and that he and his wife were working on a divorce.

“He didn't have a wedding ring on his finger,” she told authorities in a recorded interview. “He didn't even mention his kids right away either. And then one day he told me that he had two kids. … I thought it was kind of cute.”

Their relationship turned serious quickly, according to Kessinger, becoming intimate almost immediately after Shanann Watts left for North Carolina.

Kessinger told police they were having sex three or four times a day at the beginning of the relationship.

According to court documents released last week, Chris Watts appeared to be falling hard for her. He had done a Google search for the phrase, “When to say ‘I love you’ for the first time in a new relationship,” court documents showed.

Kessinger told police he told her that he loved her, and she told him the same “a couple times.”

Court documents showed that their blossoming romance featured a weekend getaway of camping and hiking, and even love letters.

Photos and videos from the escapade, as well as nude images of Kessinger, were hidden in a secret app camouflaged as a calculator in Chris Watts’ phone.

Sandra Rzucek said her daughter Shanann Watts “was clueless” to the affair.

While Chris Watts was carrying on the affair, Shanann Watts was buying self-help books and planning a private getaway to repair their marriage, according to prosecutors.

The night she was to return home from her training in Scottsdale, a storm delayed her flight. Shanann Watts enjoyed a final dinner with her closest friends, seeking advice and support on a marriage that she would never get the chance to work on.

That was the last time many of them would see her alive. The very next day, Shanann Watts and her daughters’ disappearances would make headlines across the country.

Two days later, Chris Watts was arrested and later pleaded guilty to all of the charges against him: five counts of murder in the first degree; three counts of tampering with a deceased human body; and one count of unlawful termination of pregnancy. He was sentenced in November to five life sentences without the possibility of parole, three to be served consecutively.

ABC News' Kelley Robinson and Taigi Smith contributed to this report