That's one theory being pursued by the Food and Drug Administration as it investigates how the chemical, melamine, contaminated at least two ingredients used to make more than 100 brands of dog and cat foods.
In California, state agriculture officials placed a hog farm under quarantine after melamine was found in pig urine there. Additional testing is under way to determine whether the chemical was present in the meat produced by American Hog Farm in Ceres since April 3, the state Department of Food and Agriculture said.
So far, melamine's been found in both wheat gluten and imported from China. Media reports from South Africa suggest a third pet food ingredient, corn gluten, used in that country also was contaminated with melamine. That tainted ingredient has not been found in the United States, according to the FDA, which is maintaining a pet food recall website with advice for worried pet owners.
FDA officials say they need to inspect the plants in China. But the Chinese government has not yet let them in, CBS News correspondent Nancy Cordes reports.
"Melamine was found in all three of those - it would certainly lend credibility to the theory that it may be intentional. That will be one of the theories we will pursue when we get into the plants in China," Stephen Sundlof, the FDA's chief veterinarian, told reporters.
Chinese authorities have told the FDA that the wheat gluten was an industrial product not meant for pet food, Sundlof said. Still, melamine can skew test results to make a product appear more protein-rich than it really is, he added. That raises the possibility the contamination was deliberate.
"What we expect to do with our inspections in China will answer some of those questions," said Michael Rogers, director of the division of field investigations within the FDA's office of regulatory affairs.
Still, scientists examining the tainted products say even high levels of melamine should not have killed cats and dogs — and for that matter shouldn't be harmful to humans, Cordes reports. They're now examining what happens when the chemical breaks down.
Wilbur-Ellis Co., the U.S. importer of the tainted rice protein, said Thursday it was recalling all the ingredient it had distributed to five U.S. pet food manufacturers. The San Francisco company in turn urged its customers to recall any products that may be on store shelves.
So far, just two of those companies have done so: Natural Balance Pet Foods and Blue Buffalo Co.
Natural Balance, of Pacoima, Calif., announced a limited recall Monday of its Venison and Brown Rice canned and bagged dog foods, Venison and Brown Rice dog treats and Venison and Green Pea dry cat food.