BTK serial killer's daughter shares letters he wrote her from behind bars: 'The dark side took me away' “I had to learn how to grieve a man that was not dead," his daughter told 20/20.

After "BTK" serial killer Dennis Rader’s shocking confession to murdering 10 people, which brought an end to a decades-long mystery and reign of terror in Wichita, Kansas, his daughter had to find ways to cope with her new, shattered reality. While one nightmare was ending for the town, the personal nightmare for Kerri Rawson was just beginning.

"I had to learn how to grieve a man that was not dead, somebody I loved very much that no one else loved anymore," Rawson told "20/20" in her first television interview.

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Rader, now 73, was arrested in February 2005 and pleaded guilty on June 27, 2005, to 10 counts of first-degree murder. He is currently serving 10 consecutive life sentences without the possibility of parole.

Rader's killing spree began in January 1974, when he targeted four members of the Otero family, killing Joseph and Julie Otero and two of their five children. He killed 21-year-old Kathryn Bright later that year and his next two victims, Shirley Vian and Nancy Fox, in 1977.

Rawson was born the following year.

In April 1985, Rader murdered his eighth victim and neighbor, Marine Hedge, who lived just six doors down. Vicki Wegerle became Rader’s ninth victim in September 1986. Five years passed, and then in January 1991, Rader murdered his 10th victim, Dolores Davis.

While Rader was in jail awaiting sentencing, Rawson told "20/20" during an interview that was first broadcast in February that a pastor at their church encouraged her to write to her father.

Rawson described some of the letters she exchanged with her father in her new book, "A Serial Killer’s Daughter: My Story of Faith, Love, and Overcoming."

According to her book, in the months leading up to his plea, Rawson and her father exchanged letters about his court proceedings, reading the Bible and how their family was doing. Rader often wrote matter-of-factly about what his daily life was like behind bars. He also spoke longingly about wanting his family to write him and used the words "forgive me" several times.

"You will always be my baby girl I raised right-proud-independent and now is a grown adult with many years of love to give," Rader wrote in a letter from July 2005, according to her book. "Life before the arrest was a good time and the dark side took me away."

In one letter dated April 23, 2005, Rawson asked if something had happened to her father in his childhood, searching for any explanation for his murders. Authorities said Dennis Rader has always maintained that he did not experience physical or sexual abuse as a child, a hallmark characteristic of serial killers. She also told him in that letter to "take care" and "try to stay strong and healthy."

"I wasn't corresponding with BTK. I'm never corresponding with BTK," Rawson told "20/20." "I'm talking to my father. I'm talking to the man that I lived with and loved for 26 years. ... I still love my dad today. I love the man that I knew. I don't know a psychopath... That's not the man I knew and loved."

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Rawson couldn’t bring herself to attend her father’s court appearances, and after his plea and his sentencing in August 2005, "I shut down," she said.

"I was mad. I was done. I wiped my hands of him for two years," Rawson said.

Even though she didn’t write her father again until 2007, Rader continued to write her after he was sentenced.

According to Rawson’s book, in a letter dated Sept. 22, 2005, Rader again asked Rawson how she, her husband and her brother were doing. He wrote that he wished his wife would write him – Rawson’s mother had been granted an emergency divorce in July 2005 -- and he continued to describe his routines in prison.

In that letter, he warned his daughter and her husband to "be extra careful due to all my crimes."

"I would wish no harm to you but some crazy individual might try something," Rader wrote.

In another letter Rawson includes in her book, dated Nov. 17, 2005, Rader described watching a Kansas sunset from his cell window.

"Have a west window, looks past home. Can watch bird at times and the seasons change," Rader wrote. "Kerri, you were always like that, watched and appreciated nature to its fullest. So many people never slow down to enjoy like so simple, beautiful treasures."

Rawson still didn’t respond.

"My hope is you will write me someday," the letter continued. "My love as a Dad is still there... if betrayal is what is keeping you from writing, please forgive me."

According to Rawson’s book, the following month, Rader wrote in a letter to her dated Dec. 17, 2005, how grateful he was for his children and acknowledged that this "cannot be a very 'Merry Christmas' or a 'Happy Holidays' due to my and family circumstances."

"Hope things are okay there. Snowing here today! Blessed and better 2006," Rader wrote.

When she finally wrote back in 2007, it was to let her father know she was pregnant with her first child -- a daughter.

"You were a good Dad most of the time and raised us well, and we do not know what to believe – who you were to us, or who you were to others," Rawson wrote her father in a letter dated Aug. 8, 2007.

But then Rawson said she cut off communication with him again for five years afterward. She now also has a son in addition to her daughter. Rawson said she began writing her father again in 2012, and still does to this day, because she has forgiven him.

"It was a very long journey," Rawson said. "There was a lot of hard work in me, with faith. I had gone back to church. I was working on my relationship with God, working on my own heart," Rawson said.

"I realized I was rotting within. I didn’t just forgive my father for him. I had to do it for myself."