I felt a great responsibility to know as much as I could about the actual events. Not that anyone will actually know the truth; those involved who are alive give widely varying accounts. But to write a novel based on a story, you have to know everything about that story.
I was living in Italy when I started the book. Later, I went back. I interviewed everyone I could. I read the reputable nonfiction books. I read the translated testimonies. It was a bit of a rabbit hole. Yet once I had a very good handle on the various accounts, I felt my responsibility to adhering to the facts ended. I became an “expert”, and then wrote and wrote until the characters felt real in my own mind. The only truth I was interested in was emotional truth. Once I got there, I kept going.