TAMPICO, Ill.—A summer rite of passage for tens of thousands of Midwestern teenagers turned to a time of mourning for two 14-year-old girls—neighbors and best friends—who were electrocuted in a muddy field as they removed tassels from corn.

The teenagers, Hannah Kendall and Jade Garza, were killed Monday after they came in contact with irrigation equipment or a nearby puddle conducting high voltage. Authorities said the incident was under investigation, but local farmers blamed irrigation system damage caused by a weekend...