Last month, I joined about 15,000 other cyclists in a weeklong bike ride across Iowa. We rode past cornfields and through small towns, which welcomed us with pork barbecue, slices of pie and all the trappings of a rolling state fair. Every so often, alongside the lemonade stands or in people’s yards, I spotted posters. They featured the smiling face of a young woman named Mollie Tibbetts. She was missing, the posters said. Please call with information. Tibbetts, a 20-year-old University of Iowa student, went out for a run on July 18 in her hometown of Brooklyn, Iowa, and never...