Diamond Pet Foods, the maker of a dog food that became tainted, killing dozens of dogs, noticed it was suddenly receiving more contaminated corn a month before some apparently eluded testers and got into the food, a company executive said.

Diamond said employees were warned to test closely for the naturally occurring toxin called aflatoxin, but the sampling methods probably missed a pocket of it in one of the shipments, said Mark Brinkmann, the company's chief operating officer.

The deaths of dozens of dogs nationwide have been linked to aflatoxin poisoning from contaminated food manufactured by the Meta, Mo.-based company at the South Carolina plant. The toxin is know to cause liver damage in pets.

The company typically rejects one or two truckloads of corn each year because of aflatoxin levels exceeding federal limits, Brinkmann said. But in September, the company began identifying the fungus on one or two truckloads of corn each week and rejecting the loads, he said. The Gaston plant receives about 12 shipments of corn a week, he said.

Diamond officials now believe contaminated corn entered production in early October. A new test was implemented Nov. 30, before the company realized contaminated food already had been shipped.

"I have no reason to believe that the test failed," Brinkmann said.

The company issued a wide recall in December, after a New York veterinarian linked a dog's death to the company's food.

On Wednesday, the company narrowed the batches of potentially toxic dog food to two varieties: Diamond Maintenance Dog and Diamond Premium Adult Dog with "Best By" dates of April 3, 4, 5, and 11, 2007. But the company said consumers should continue to return to retailers any products included in the initial recall, which included 19 different varieties of pet food.

The company's new testing method provides more information but still relies on sampling, Brinkmann said. "There is no foolproof way to do it."

The company also began checking the finished product for aflatoxin Sunday, he said.