Chris Watts told his pen pal how he "daydreamed" about killing his wife, Shanann, reports state.
The family killer made the confession in a 12-page letter he sent to 67-year-old Cherlyn Cadle, which left the gran of 12 in tears, reports the MailOnline.
Watts admitted strangling his pregnant wife, Shanann and his two daughters, Bella, four, and Celeste, three, in August 2018.
Shanann was found in a shallow grave in Colorado, US, with his daughters being discovered stuffed inside an oil tank an hour's drive from their family home.
The story shocked Netflix viewers in documentary "American Murder: The Family Next Door" with Watts repeatedly maintaining his innocence and even made several TV appeals for them to return.
He would admit his sick crimes when his cover-up started to unravel when cops discovered he was having an affair with Nichol Kessinger.
Nichol had fallen for "softly spoken" father-of-two Watts when the two began working together.
The twisted killer was sentenced to five consecutive life sentences for the brutal slayings. In exchange for pleading guilty, prosecutors agreed not to pursue the death penalty.
In a new episode of Lifetime's Cellmate Secrets, Cadle said she began writing to Watts after seeing his first TV interview. He wrote back from his prison cell after her third letter.
"He told me he would daydream about killing Shanann,' Cadle said.
"She would be yelling at him or be upset about something and he wouldn't fight back, but he would just stand there and just daydream about what it would be like to kill her."
Speaking to Fox News, Cadle said she " just broke down and started bawling" when she first read the letter.
"I cried and cried. I just couldn’t believe what he did to those girls… It was a real shock to read his letter."
Last month, The Mirror reported how Watts had been shunned in prison and had been put in protective custody to stop him being attacked.
He is being held in maximum security Dodge Correctional Institution in Waupan, Wisconsin, US.
A source who regularly speaks to Watts told People magazine in the US that he can go days without speaking to a soul as inmates and guards try to avoid him.
The source said: “No one wants anything to do with him. He’s on the lowest social tier of the entire prison."
- The Cellmate Secrets episode, narrated by Angie Harmon, airs on Friday at 10pm EST.
