How could you not have known? It’s the question Kerri Rawson gets asked most often after her father was revealed to be one of the country’s most notorious serial killers.

How could she not have had an inkling that Dennis Rader, her father, was “BTK” — short for bind, torture, kill — a murderer who stalked the streets of Wichita, Kan., from the 1970s to the 1990s?

And Rader even kept mementos from his crimes in his home. They were there for his family to find. Hidden inside a book was a business card with the methodology of his next kill scribbled on the back. Stuffed inside a backyard shed was underwear stolen from his victims.

Only Rawson wasn’t in the habit of rifling through her father’s things. She would have been yelled at.

And even those closest to us often have an unknowable side.

“I’ve always argued he’s 95 percent my dad and 5 percent I don’t know — don’t know that man. Never met him,” Rawson has written. “I’ve still not reckoned fully with who my father really is.”

Her attempt comes with a new book, “A Serial Killer’s Daughter: My Story of Faith, Love, and Overcoming” (Nelson Books).

It begins on Feb. 25, 2005, with an FBI agent appearing at Rawson’s door. Her first thought was that she had accidentally downloaded something she shouldn’t have. Instead, the G-man had a bombshell about her seemingly normal father.

The revelation dropped Rawson, then 26, into a dark hole of denial, depression and anger.

Learning that her father was leading a double life as a man who’d murdered 10 men, women and children forced her to revisit the past and question nearly everything about the man she thought she knew.

As a little girl, Rawson could remember her father carrying a maroon bowling bag with a white stripe. Was it the same one in which he’d carried a “hit kit” to break into the house of Marine Hedge and strangle her in 1985?

Was the transistor radio that had sat on her father’s bedside for years the same one that he’d apparently stolen from the Otero house, in which he’d butchered four family members in 1974?

Rader had taken his daughter to see the movie “Seven” in 1996. Was he trying to tell her something, she wondered?

Like most serial killers, Dennis Rader seemed like a regular Joe. He attended church, raised a family and collected stamps.

He was born in 1945 and grew up in Kansas. He married Paula Dietz in 1971 and eventually had two children, son Brian in 1975 and Kerri in 1978.

Rader killed his first victims in January 1974, before Rawson was born. He forced his way into the Otero home, then bound and murdered the parents and two children. A few months later, he hid in the closet of 21-year-old Kathryn Bright, popping out when she returned home. He tied her up and stabbed her to death.

He killed five more times between 1977 and 1991.

Rader fed off the publicity the crimes generated, and over the decades, he sent letters to the media and left clues in public places, including a detailed description of the Otero murders inside a book at the public library.

In 2004, 30th-anniversary coverage of the first BTK murder spurred Rader to begin communicating again. It would be his undoing.

He mailed a computer disk to a TV station in February 2005, and metadata analyzed by the police pointed to Rader. The authorities quickly arrested him.

Rawson at first thought there’d been some mistake. Maybe her father was trying to solve the BTK case and got caught up?

She Googled more about the murders, and “with every click, scroll and news article, I tumbled into an abyss of despair and terror,” she writes.

Rawson remembered a night in 1985 when she was 6 years old.

“It stormed that night. I was scared. Thunder rattles our home — sometimes the house shakes. I crawled into bed with Mom. I wouldn’t have if Dad was home. Slept on his side of the bed. Did that sometimes when he was gone. I only remember that night because our neighbor lady went missing.”

That neighbor was one of her father’s victims.

She came across two suspect sketches from 1974 that looked vaguely like her father. And then there was a 911 call the killer had placed in 1977 after strangling Nancy Fox.

“Through the static, in seizing fear, I recognized my dad’s voice — younger, but him,” she writes. “Everything I’d ever known, loved, believed was falling down around me. My whole life was a lie — from before I was born.”

The case garnered massive media attention, and reporters — plus everyone from Oprah to Larry King — began hounding Rawson for an interview.

She and her family refused.

Rawson, married and living in Michigan, worked as a substitute teacher and was never in one school long enough to make close connections.

Although she still remained somewhat anonymous, her identity had nonetheless changed.

“I’m BTK’s daughter,” she writes. “No longer Kerri — she’s gone.”

Initially, Rawson, her mother and brother kept in touch with Rader, writing him letters in jail. They urged him to plead guilty and struggled to understand what kind of monster lived within him.

“We still love you. We love the husband, father and man we know with all our hearts,” Rawson wrote to him on March 12, 2005. “We understand there is something seriously and deeply wrong with . . . you.”

Rader doesn’t offer much clarity.

“I do have some serious problems, and I do need help on them. Need your prayers and thoughts on that,” he responded to Rawson in March 2005.

It’s about as close to an answer as Rawson would get. Her father continued sending letters, but they are creepily mundane.

“I hope the Tempo drove okay,” he wrote in one missive. “Oil that it uses should be in the glove compartment.”

Rader pleaded guilty and was sentenced to 10 life sentences in 2005. Rawson, fearing the media circus, did not attend the court proceedings. Her mother was soon granted an emergency divorce, and the house where Rawson had grown up, and where Rader had planned the murders and stored souvenirs, was sold and ultimately demolished.

Rawson and her family remained torn, stunned by the mountain of evidence against Rader but still holding on to the memory of the man they had known.

“Does that make me a bad person — to enjoy eating a really good hamburger and getting pleasure from the thought you won’t ever get to?” Rawson wrote in a 2006 letter to her father. “Or should you know in the next breath I want to ask if you’re staying warm at night, did you get some house slippers and an extra blanket or two?”

‘Everything I’d ever known, loved, believed was falling down around me. My whole life was a lie — from before I was born.’

Rawson and her family were especially wounded when, during one of Rader’s rambling court appearances, he’d minimized his family as “social contacts.”

The slight ultimately led Rawson to sever contact with her father. She went through her photo albums and stuffed every picture with him into a trash bag, topped it with kitty litter and tossed it.

After her father’s arrest, Rawson suffered paranoia, night terrors and depression. She began seeing a therapist in 2007 and eventually worked through her issues.

Then in 2012, when Rawson was driving home from a movie, “forgiveness toward my father unexpectedly washed over me while I was sitting at a red light,” she writes.

She drove home and sat down at her desk and wrote the first letter to her father in five years, re-establishing contact (although she still hasn’t personally visited him).

“What’s in my past is what it is; it can’t be changed — Dad murdered 10 people and devastated countless lives,” she writes in her book. “Yet on the days when I’m not wrestling with hard, terrible truths, I will tell you: I love my dad — the one I mainly knew. I miss him.”