Joanna Sullivan was always haunted by the memory of that day.
The day she learned a girl about her age had disappeared in their quiet suburb of Broomall, Pennsylvania.
It was 1975, and Sullivan was 9 years old.
"Her body was not found two months after she disappeared. And I think for a long time people were scared, but, you know, life goes on. But people never forget," said Sullivan.
Even in her adult years, the unsolved mystery nagged at her.
She reconnected with an old friend, Mike Mathis, at a high school reunion, and the pair decided to investigate.
"Generations of detectives had tried. They kept on looking at the files: What did we miss?" Sullivan said.
After combing through old paperwork, and interviewing dozens of people, Sullivan and Mathis wrote a true crime book all about the murder of Gretchen Harrington.
Just nine months after the book was published and 48 years after her death, police made an arrest: Reverend David Zandstra.
"He is every parent's worst nightmare. This is a man who is remorseless child predator, who acted as if he was a friend, a neighbor, and a man of God," said Jack Stollsteimer, district attorney for Delaware County, Pennsylvania.
Suddenly, everything clicked for Sullivan.
She had interviewed Zandstra months prior, knowing he was close friends with young Gretchen's family and a pastor at her Bible camp.
"He seemed like he didn't really remember a lot of that morning," Sullivan said. "His wife did most of the talking. And we chalked it up to old age. We thought, oh he's 82 at the time we talked to him, He doesn't remember much."
But in his confession to police, he remembered everything. How he lured Gretchen into his car, drove her into the woods, and asked her to remove her clothes.
When she refused, he beat her to death.
All of this came to light thanks to a tipster who came forward earlier this year, telling police that the pastor had molested her and that she always believed that he had killed Gretchen.
"I've been told that this definitely brought up some memories for people and perhaps prompted her to come forward," Sullivan said.
Now the author is hearing from other potential victims. People who may have narrowly avoided the same fate as 8-year-old Gretchen Harrington.
Sullivan and Mathis are going to update the book with the new information about Zandra's arrest.
She also says this will likely not be her last time looking at a cold case.
