Judy Belfrey. Kathy Williams. Holly Davidson. Melodie Stankiewicz. Four young women whose lives crossed the same few blocks of Boston in the 1970s, and whose deaths left investigators with a pattern they could see but never fully explain.

If there is any case where you wish you could turn back the clock and change just one small decision, it might be the disappearance of Bianca Lebron. There were several moments that morning in November 2001 when a question, a phone call, or one person stopping her before she walked away might have changed everything that came next.

One night in October 2006, a college student needed to borrow a phone. She happened to ask a stranger with violent intentions. What makes that even harder to sit with is what investigators learned about this man afterward. There were red flags everywhere, allegations that reached back years, but none of it stopped him from being there that night.