November 28, 2018 08:00 AM

To the outside observer, Chris Watts appeared to be a doting husband and father who loved being around his family. Living in an upscale home in Frederick, Colorado, he seemed upbeat in online videos with his wife, Shanann.

“I thought Chris was such a good dad,” his neighbor, Kelley Trippy, tells PEOPLE in its upcoming issue. “It was like he was a jungle gym for the kids.”

But there was another side to Watts that even those closest to him didn’t know. His double life was exposed when he was charged with the August murders of Shanann 34, who was 15 weeks pregnant, and their daughters, Bella, 4, and Celeste, 3. The brutality of the crime — to which Watts later pleaded guilty and received a life sentence in prison without parole — shocked even the most seasoned investigators.

“This was the most horrific case that I’ve ever done,” Weld County Prosecutor Michael Rourke tells PEOPLE. “I think you combine the horrible facts of Shanann being pregnant and two little girls being killed as well, and then the horrifying condition in which their bodies were disposed — I think all those things together just sickened everyone who had anything to do with it.”

Weld County District Attorney

RELATED: Chris Watts’ Mistress Breaks Silence About Affair with Triple-Murderer: ‘He Lied About Everything’

So how did this seemingly doting dad turn out to be in the words of Shanann’s father Frank Rzucek a “heartless monster?”

In early July, while Shanann was visiting her parents in North Carolina, the 33-year-old began having affair with a coworker, Nichol Kessinger. Prosecutors later cited Watts’ desire for a “fresh start” with Kessinger as his motive for strangling his wife and smothering his daughters.

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While Watts was having an affair, Shanann was desperately trying to salvage their relationship by pleading with him to be less distant and giving him books on self-help and relationship counseling, according to prosecutors and more than 2,000 pages of documents released by the Weld County District Attorney’s Office after Watts’ sentencing.

But Watts had mentally checked out of the marriage and ignored Shanann, prosecutors said, even throwing one of the books in the garbage.

RELATED: Behind Chris Watts’ Decision to Plead Guilty — ‘Eventually He Acknowledged His Life Was Over’

Chris Watts
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After Shanann and their daughters went missing, Watts’ behavior was troubling. According to computer searches, he looked up the lyrics to Metallica’s song “Battery.” The track contains lyrics such as “Smashing through the boundaries, lunacy has found me” and “Pounding out aggression, turns into obsession/Cannot kill the battery/Cannot kill the family.”

Authorities say he later called his girls’ school to say they would not be attending the upcoming year. He also contacted his Realtor about selling his home.

After the shocking violence, those close to the couple are left searching for answers about a man they thought they knew.

“I am still struggling to understand how and why this tragedy occurred,” Watts’ mother, Cindy, said in court at Watts’ sentencing. “I may never be able to understand and accept it, but I pray for peace and healing for all of us.”

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