SAVANNAH, Ga. — Behind the immaculate gray walls of the Customs and Border Protection’s laboratory here stands a cabinet containing three plastic vials filled with a sticky, yellowish substance. Honey, or so an importer has claimed.

The lab’s task: Determine whether the samples are adulterated with sweeteners or syrups, and, if they really are mostly honey, figure out where it originated. If the honey comes from China, often the case, the entire shipment from which the samples came may be subject to additional taxes.

The chemists here regularly test a wide range of imported goods, but they specialize in analyzing agricultural imports. With remarkable precision, these scientists can tell you where the peanuts in your peanut butter came from and where the mangoes in your jam were grown.

But honey, No. 0409 on the 2015 Harmonized Tariff Schedule, has been a focal point for the lab and the source of a long-running international food scam that has challenged even the existing forensic technology.