Chris Watts shocked the world when it was revealed the "perfect" family man was actually the killer behind the murders of his wife and two daughters.
At first, the 34-year-old Colorado man denied any wrongdoing, even going on live TV shortly after burying their bodies, begging anyone with information to come forward, reports News.com.au.
But investigators would soon uncover the father-of-two, with a son on the way, was actually the man behind the brutal murder of his own family, simply by how he referred to them.
Watts is serving a life sentence in the US for killing his wife Shanann, 34 – who was carrying his unborn son, Nico – and two daughters, Bella, four, and Celeste, three, so he could be with his lover, Nichol "Nikki" Kessinger.
• 'Cold' video footage of Chris Watts after he killed whole family revealed
• Killer dad Chris Watts: 'If I hadn't met Nikki, I would never have killed my family'
• Killer dad Chris Watts ordered to pay $9.3 million to murdered wife's parents
In a new documentary, Criminal Confessions, which aired in the US over the weekend, police revealed how they knew he was the culprit, detailing the perfectly timed strategies they used to lock in Watts' murder confession.
Apart from his emotionless and "cold" behaviour during his first news interview, investigators said red flags were raised in how Watts referred to his family.
He kept describing Shanann and his two little girls in the past tense.
Not only did Watts say Celeste "loved" a certain pair of shoes, but he said his now deceased four-year-old "was a girly girl", People magazine reported.
"If he is hoping and believing that they're still alive, so why would you start talking about them in past tense?" Colorado Bureau of Investigation (CBI) agent Tammy Lee said in the 90-minute doco.
A lie-detector test also exposed Watts' affair with his co-worker Nichol Kessinger, something he had previously denied.
"I hope that, you know, if you did have something to do with their disappearance … it would be really stupid for you to come in and take a polygraph today," Mr Lee said to Watts before the polygraph.
After a line of questions, it was ruled that Watts had failed the polygraph test, and he later confessed to having an affair.
But investigators needed more, they needed him to confess he was behind his family's deaths, and that's when they began to paint Shanann as the villain, a technique Mr Lee said investigators used to eventually get the truth.
"We just needed to find Shanann and the girls," Mr Lee said.
A deflated Watts then asked to speak to his father – it was a risk, but Lee and fellow FBI agent Graham Coder allowed the conversation to take place, and that's when they got a partial murder confession from Watts.
"It was pretty much gut-wrenching to hear Chris talk about the fact that he murdered his wife to his own father," Mr Lee relayed in a confessional. "It was the theme of Shanann doing something to the girls that I had just given him prior to leaving the room. But to actually hear that come out of his mouth was shocking."
According to People, upon hearing Watts' confession, investigators re-entered the room and pressed for details about the missing Watts' whereabouts. Eventually, Watts admitted to burying his wife in a shallow grave and placing his daughters' bodies in the nearby oil and battery tanks.
"Chris told us that, every night when he closes his eyes, he hears Bella yell, 'Daddy no!'" Mr Lee concluded. "I feel like it's kind of what he deserves. I hope he hears that every night."
Watts was sentenced to life in prison for the August 2018 murders of his family.
In prison letters sent to Cheryln Cadle, an author behind the explosive new book Letters From Christopher, the family killer lays out how he smothered his two daughters and wife Shanann at their home in Frederick, Colorado.
"After Shanann had passed, Bella and Cece woke back up. I'm not sure how they woke back up, but they did. Bella's eyes were bruised, and both girls looked like they had been through trauma. That made the act that much worse knowing I went to their rooms first and knowing I still took their lives at the location of the batteries," Chris wrote.
According to the letters, Watts had been plotting the murders for some time, and it was not a spontaneous outburst as he previously claimed.
He was having an affair with his co-worker Ms Kessinger and explained how if he didn't kill Shanann, she would keep him from being with his mistress.
"I knew if I took my hands off of her, she would still keep me from Nikki. They asked me why she couldn't fight back, it's because she couldn't fight back," Watts wrote in the letters.
Bella and Celeste came into the room asking what was wrong with their mother, to which Watts told them she wasn't feeling well.
"The girls were just kind of running around the house and watching me with scared looks on their faces. Bella started to cry and when she did, Celeste started whimpering. What a nightmare this was," Watts told Cadle.
He would later murder both his girls and dump their bodies in oil tanks on the property where he worked.
"Out of all three, Bella is the only one that put up a fight. I will hear her soft little voice for the rest of my life saying, 'Daddy, NO!!!' She knew what I was doing to her. She may not have understood death, but she knew I was killing her," he told the author.
In November this year, the convicted family killer was ordered to pay $US6 million ($NZ9.15 million) to the parents of his wife Shanann.
Shanann's parents filed the lawsuit on the same day Watts pleaded guilty to collect any money he might have and prevent him from profiting from the murders should he ever decide to write a book or sell the rights to his story, the Denver Post reported.
$US1 million ($NZ1.5 million) from the $US6 million is for each of the deaths and $US3 million ($NZ4.5 million) is for emotional pain.
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