ATLANTA — Federal inspectors on a Delta Air Lines plane say they found so many rat droppings that they lost count.

The droppings were found near the plane’s galley during an inspection at the Atlanta airport in late January. Droppings were found above the door panels near the galley, where food is prepared. And inspectors found a large number of droppings in the ceiling panels above a second galley and above five seats.

The discoveries prompted a warning letter from the Food and Drug Administration to Delta on April 13.

The FDA said the plane where the droppings were found was the only one it examined during an inspection that lasted from Jan. 26 to Feb. 2. The FDA said in its letter that Delta’s response did not include actions it will take to prevent future infestations. “We believe a recurrence is likely without adequate preventive measures in place,” the FDA wrote.

Delta said it was an isolated incident and that the plane was pulled from service and cleaned.

The FDA regularly issues warning letters to companies that don’t follow regulations for manufacturing and marketing drugs, food and other products. The letters are not legally binding, but the FDA can take companies to court if they are ignored.