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JOHNSON COUNTY, Ind. — The children fell ill, one by one, with cancers that few families in this suburban Indianapolis community had ever heard of. An avid swimmer struck down by glioblastoma, which grew a tumor in her brain. Four children with Ewing’s sarcoma, a rare bone cancer. Fifteen children with acute lymphocytic leukemia, including three cases diagnosed in the past year.

At first, families put the illnesses down to misfortune. But as cases mounted, parents started to ask: Could it be something in the air or water?

Their questions led them to an old industrial site in Franklin, the Johnson County seat, that the federal government had ordered cleaned up decades ago. Recent tests have identified a carcinogenic plume spreading underground, releasing vapors into homes.