ST. PAUL — On a fall day in 2015, Madeleine Baran and Samara Freemark went for a walk through the extensive skyway that threads through the buildings of downtown St. Paul.

They were new colleagues at APM Reports, a division at American Public Media where investigative reporters and radio producers had been “smushed together,” as its editor in chief, Chris Worthington, put it.

Strolling above the city, Ms. Baran and Ms. Freemark talked about ideas for their first project. Ms. Baran, a reporter who had grown up mainly in Milwaukee, had lived in Minnesota long enough to be familiar with the case of Jacob Wetterling, an 11-year-old who had been abducted from the nearby town of St. Joseph in 1989. How, she wondered, had the crime gone unsolved?

Ms. Freemark, an audio producer from Durham, N.C., said the story might lend itself more to a podcast than to a radio documentary. The megahit first season of “Serial” had recently completed its run, proving there was an audience for an audio project that unspooled over many episodes. Ms. Baran agreed.