A year and a half after Mary saw the man at her window, in the spring of 2001, a reporter friend of mine sent me a fax about the arrest of one William Joseph Green. He lived in Eugene, Ore., the small city where I was raising my family. He had turned in a roll of film for processing that contained images of a young girl lying on top of him; when the police searched his house, they discovered dozens of troubling videotapes. I called the police, and the following day, I met with a detective who showed me a stack of still images captured from the videos. The third one was of Mary. I didn’t have to go on, and didn’t, but the detective wondered if my daughter would look at all the pictures and help sort out the identities of the other girls.

This moment was the crux, the Y in the road. I could have refused the detective, gone home and said nothing to my girls. We could have forged on, at least for a while, in the seductive comfort of avoidance. It was tempting to believe that Mary would move on, soon forgetting about the man at the window. But would my daughter forgive me if she discovered years later that I had learned his identity and thwarted her chance to be involved in bringing him to justice?

I did tell Mary about the arrest and the detective’s request, a choice I have sometimes regretted. I remember the sores around her fingernails, her chewed-up flesh, which made me want to keep her at home, away from any police station or courthouse. But Mary stared me down, determined to let other girls find out, too, that William Green had been caught.

The detective came to our house to lay out the photos, cut so that only faces showed, on our kitchen table. Some of the girls were unaware that they were being taped, delighted in that crystallized moment in their rooms, while others seemed to sense a presence outside their windows. Their expressions, to my eyes, betrayed a terrible fear.

Mary recognized in the images her younger sister — a devastating discovery — and also about 10 other girls she knew, from school, from dance class, from hanging around a neighborhood that would never again feel safe to her. Although many residents had filed peeping-Tom complaints with the police over the years, no officer had ever linked those reports. Mr. Green was caught only after he dropped off the film for developing. The girl in those photographs turned out to be 11 years old.