Detective John Williams says investigators found a shotgun and the rifle Sunday morning, side by side on the passenger-side floorboard of a stolen farm truck, barrels pointed at the driver.
Williams says it appears that a lever on the shotgun got into the trigger guard of the rifle and fired it.
The weapons were among other goods taken from a nearby house on the farmland.
Williams says detectives found no indication anybody else had been in the truck, whose engine had been running for hours.
The man was identified as 19-year-old Genaro Hernandez Mendoza.
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