A hazmat team was called to the post office in Firth early Saturday after a white powdery substance was found.
Bingham County Sheriff Craig Rowland said it turned out to be corn starch. As a precaution, everyone who was exposed was told to stay inside the post office until the hazardous material team figured out what the substance was.
The main road near the post office was blocked while an investigation took place. Rowland said the FBI is now investigating the incident.