Chelsea Cain’s Heartsick, the first book in a new thriller series to feature Portland, Ore., police detective Archie Sheridan, introduces an unforgettable—and frighteningly plausible—serial killer, Gretchen Lowell.

When you were developing this book, which did you create first, the detective or the serial killer?

I developed them simultaneously because I was interested in their relationship—a cop who is more or less “in love” with a serial killer.

Do you have Gretchen’s background all sketched out? She teases Archie by mentioning incest and a horrible childhood but not much beyond that.

I have it worked out, but I don’t want to reveal much about it yet. Suffice it so say, don’t trust anything Gretchen says about anything.

Like Susan Ward, the reporter in Heartsick, you had a “countercultural” childhood.

I spent my early years on a hippie compound. My parents didn’t have jobs, drivers’ licenses, a telephone or a working toilet. My dad was resisting the Vietnam draft. My mom was running away from the expectations of her Catholic upbringing. Their friends came to their farm one by one and never left. Soon there were about 14 adults (more or less) and, eventually, me. I was the only kid. Unless you count the goat offspring.

What mystery writers do you read today?

Val McDermid, a writer from Scotland, is one of my favorites. But right now I’m immersed in forensic pathology and criminology books.

What were you doing in your nonwriting life before you tackled this book?

I was a creative director at a PR firm, and my husband was running our video store. Then I took a year off to work on Confessions of a Teen Sleuth. Heartsick was the book I worked on when I was supposed to be working on Confessions.

Portland as a setting is relatively virgin territory for a series.

Yes, and since I live in Portland it was a lot easier to write a book that takes place here rather than, say, Detroit. Plus, the Northwest has had more than its share of serial killers. When I was growing up in Washington State, the Green River Killer was at large, and there was a task force assigned to catch him. Elements of Heartsick are based on that chase.

Were you interested in real-life mysteries growing up?

When I was a senior at high school, an 18-year-old girl disappeared from a nearby town. I didn’t know her, but I had friends who did. She went out jogging with her dog, and the dog came back several hours later without her. They found her body near a creek. She’d been raped and strangled. Crimestoppers came to town to film a re-enactment, and they asked me to play her because we looked so much alike. I went to her house and met her mother and the dog she was jogging with when she disappeared. I saw the place where they found her body. When I was writing Heartsick, I tried to think of a scary way to die, and that’s what came to me. The image of this girl, dead by that creek. So the After School Stranger rapes and murders teenagers and leaves them by water. They never found the girl’s killer. So I made one up and caught him.