Imported ingredients used in recalled pet food may have been intentionally spiked with an industrial chemical to boost their apparent protein content, U.S. federal officials said.

That is 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.

Click here for a list of some contaminated pet food brands.

In California, state agriculture officials placed a hog farm under quarantine after melamine was found in pig urine there. Additional testing was 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 has been found in both wheat gluten and rice protein concentrate 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, the FDA said.

FDA investigators, meanwhile, are awaiting visas that would allow them to visit the Chinese plants where the vegetable protein ingredients were produced.

"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 Thursday.

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 is, he said. 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.

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 California 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 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.

Blue Buffalo followed Thursday by recalling 5,044 bags of its Spa Select Kitten dry food. The company intercepted most of the kitten food before it reached distribution centers, company co-founder Billy Bishop said.

FDA officials would not release the names of the other two manufacturers that Wilbur-Ellis supplied, citing a continuing investigation.

The FDA and the Agriculture Department also were investigating whether some pet food made by one of the five companies supplied by Wilbur-Ellis was diverted for use as hog feed after it was found unsuitable for pet consumption.

"We understand it did make it into some hog feed and we are following up on that as well," Sundlof said.

The FDA could not provide updated numbers of pet deaths or injuries due to the contaminated pet food. The agency has received more than 15,000 calls since the first recall was announced more than a month ago.