Sick killer Chris Watts described how he murdered his two children Bella, aged four, and Celeste (Cece), aged three, in one of his disturbing prison letters to true crime author Cheryln Cadle.
The letter laid out how the murderer attempted to smother his two children before strangling his pregnant wife Shanann at their home in Frederick, Colorado.
However, his attempt to murder his daughters failed as they woke up bruised and traumatised from what they had endured.
The oil field operator did not confess to the first murder attempt during his multiple interviews with police and FBI agents following the murders, which took place in August 2018.
The grim and detailed letter was sent to Ms Cadle who was in contact with the killer after he was jailed, reports the Daily Mail.
She encountered multiple conversations and received numerous letters from Watts which allowed her to form the basis for her book, Letters From Christopher, published in 2019.
In the letters, obtained by the Daily Mail, Watts writes: "August 13th, morning of, I went to the girls' room first, before Shanann and I had our argument.
"I went to Bella's room, then Cece's room and used a pillow from their bed (to kill them). That's why the cause of death was smothering.
"After I left Cece's room, then I climbed back in bed with Shanann and our argument ensued."
He continued: "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."
In the letters, he adds that he was concealing other facts from authorities that he would not talk about in the phone calls from prison to Ms Cadle as he was concerned about them being recorded.
The killer previously claimed that a spontaneous outburst led him to murder his pregnant wife and his two daughters so he could be with his mistress and co-worker, Nichol Kessinger.
However, in the letter to Ms Cadle, Watts said that he had been plotting the murders for a while.
He wrote: "August 12th when I finished putting the girls to bed, I walked away and said 'That's the last time I'm going to be tucking my babies in.' I knew what was going to happen the day before and I did nothing to stop it!"
He also confessed in the note that he had slipped a painkiller to his wife in the hope of inducing a miscarriage. 'I thought it would be easier to be with Nichol if Shanann wasn't pregnant,' he wrote.
Watts appeared to have turned down multiple interviews before he was in contact with Ms Cadle and he hoped that the book could be his 'story of redemption'.
However, the author does not condone Watts' evil actions.
In her first letter to the killer, she wrote: "Let me just say, the crime was horrific, so I’m not writing you to tell you how wonderful you are or that I want to be pen pals."
She wrote a total of three letters before Watts responded and as she was writing the book, she met him multiple times at the prison.
After they met in person, Watts allowed Ms Cadle to use his letters in her true crime book.
In exchange for his approval, Watts asked the author to publish his 'testimony of coming to God and the forgiveness he received'.
In his letters, the killer quashed the rumours that Shanann was to blame for the death of Bella and Celeste, as Watts had claimed in one of his police interviews.
He also recalled how his wife looked when she was murdered as he said: "Isn't it weird how I look back and what I remember so much is her face getting all black with streaks of mascara?
"All the weeks of me thinking about killing her, and now I was faced with it."
He continued: "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."
His daughters then flooded into the room after hearing the commotion and Watts explained that their mother wasn't feeling well.
He attempted to carry Shanann's body down the stairs, draped in a bedsheet, but she was too heavy so he dragged her down the remaining steps before bundling her in the back of his truck.
Watts told the author: "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.
"I realise now the girls getting up and walking around may have been God's third attempt to stop what I was doing."
He then drove his wife's body and his two daughters to a remote oil field owned by his then-employer, Anadarko.
He told Ms Cadle: "I dumped Shanann on the ground, then I walked back to the truck and with the blanket that Celeste was holding, I put it over her head and smothered her."
The killer then squeezed Celeste's body through an eight-inch hatch in one of the oil tankers, adding to the author: "I couldn't believe how easily it was to just let her drop through the hole and let her go. I heard the splash as she hit the oil."
As the conversation took another gory turn, he relieved for Ms Cadle how he murdered his eldest daughter, Bella, after she witnessed him murder and then dispose of her sister.
He said: "Little quiet Bella had a will to live.
"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."
After he murdered his children, Watts returned to digging a shallow grave for his wife, telling Ms Cadle: "When I dug the hole, it seemed a lot deeper than it was."
Her cause of death was ruled as strangulation and the two girls were smothered.
Following the murders, Watts' lies soon began to unravel as he made a public appeal on American television for his family's safe return.
Surveillance footage from his neighbours home soon revealed that Watts loaded up his truck on the morning of the murders, before he drove off.
The killer had told police that Shanann had run off with their daughters while he was at work but the footage revealed no one but Watts had left the house that day.
Watts then took part in a polygraph test that was administered by Frederick Police, which later indicated that Watts was lying.
After a conversation with his father, Ronnie, Watts admitted to murdering his wife and initially claimed that Shanann had killed Bella and Celeste.
However, he later admitted to murdering his children.
In November 2018, Watts pleaded guilty to multiple counts of first-degree murder as part of a plea deal when the death penalty (which was later abolished in Colorado in 2020) was removed from sentencing.
He was sentenced to five life sentences without the possibility of parole, three to be served consecutively.
