A 17-year-old died in 1975. Two men were just arrested in her killing.

Laurel Jean Mitchell had just left work at a church camp in North Webster, Ind., and started the walk to an amusement park to meet friends. But she never made it to Adventureland that night in August 1975. The 17-year-old’s body was found in a river the next morning. Despite an investigation that spanned multiple Indiana agencies and counties, the case went unsolved for decades, leaving Mitchell’s family with no answers. Her cause of death was listed as drowning, but the autopsy revealed “she had fought for her life,” authorities...