In Genealogy of a Murder (Norton, May), journalist Belkin retraces the choices and chances that culminated in the 1960 murder of a Connecticut cop.

When did the seed for this book get planted?

Over breakfast in my mother’s kitchen with her brand-new husband in Tucson, Ariz., in 2014. My new stepdad, Alvin Tarlov, read my previous book, Show Me a Hero, and it reminded him of a story. It was the story of his interaction with a prisoner in 1960, and his attempts while conducting medical tests in that prisoner’s facility to help the man get back into society. It went terribly wrong, and it ended up with the murder of a cop. I said, “You know I have to write about this, right?”

You spent close to a decade writing about it. What kept you interested for so long?

What fascinated me was that my stepdad and the cop [David Troy] and this ex-prisoner [Joseph DeSalvo] all basically started at the same starting line. They all were the children of immigrant families who came to the U.S. for the same reasons. It’s been a lifelong fascination of mine: how do we become who we are? And this became a way to tell that. How did one become the cop, one become the killer, and one become my stepfather, who inadvertently set this fatal shooting in motion?

Your stepfather has had half a century to think about his vouching for DeSalvo. How does he feel about that choice today?

It’s complicated. After DeSalvo was released, he reached out to my stepfather on a day when his wife was having an emotional breakdown. Alvin had to choose: does he go see his newly released friend, or does he take his wife to the beach to get her away? He chose his wife, and it was during that time that DeSalvo bought a gun, and that led to the holdup, and that led to the murder. Alvin’s view is that he would still have worked to get this man paroled, but also worked to provide more scaffolding for DeSalvo once he got out of prison.

How do notorious murderers Leopold and Loeb factor into your narrative?

Leopold and Loeb were imprisoned for life, and they bet on whether people who were released were going to come back to the prison. They were right a lot of the time, and they started to ask why. They ended up with a theory, published in various sociological journals—under a pseudonym, because these two murderers could not publish in journals—predicting who would make good on parole and who wouldn’t. Their research led to the development of a questionnaire that became the de facto way of determining parole eligibility in Illinois, and it’s what led to the release of Joe DeSalvo before he shot David Troy.