SHANGHAI, May 2 — A Chinese company accused of selling contaminated wheat gluten to American pet food suppliers avoided inspections partly because it did not correctly disclose its shipping contents to Chinese export authorities, according to American regulators.



The Xuzhou Anying Biologic Technology Development Company, one of two Chinese companies at the center of the huge recall of pet food that has killed or sickened thousands of animals, shipped more than 700 tons of wheat gluten labeled as nonfood products this year through a third-party Chinese textile company.



By listing the goods as nonfood items, the company’s shipments were not subject to mandatory inspection by the Chinese government. Though a possible violation of export policies, such mislabeling is thought to be widespread in China.



The details of the case, some of them disclosed on Friday in a circular released by the Food and Drug Administration, are just the latest clues that Chinese feed suppliers may have been intentionally disguising the contents of their goods.



F.D.A. officials are now visiting China to seek more information about how and why an industrial chemical used in plastics and fertilizer got mixed into pet food ingredients.



American regulators admit that six weeks after one of the biggest pet food recalls in United States history, they still do not know who in China manufactured the contaminated pet food ingredients or where in China the contamination took place.



Though the agency has named two Chinese companies as the suppliers of the tainted vegetable protein sent to the United States, regulators suspect the companies may not have been manufacturing the feed, but buying it from dozens of other feed manufacturers in China.



Those feed producers, regulators say they believe, may have intentionally mixed melamine into the feed to inflate artificially the level of protein in the bags to meet pet food requirements.



“Records relating to the importation of these products indicate that these two firms had manufactured the ingredients in question,” the F.D.A. said in an import alert released last Friday. “There is strong evidence, however, that these firms are not the actual manufacturers. Moreover, despite many weeks of investigation, it is still unknown who the actual manufacturer or manufacturers of the contaminated products imported from China are.”



Worried that the contaminant may continue to enter the United States and also seep into the human food supply through food additives, regulators have blocked all Chinese imports of wheat gluten and warned importers to screen nearly every other kind of food and feed additive entering the United States from China, including corn gluten and soy protein.



Last week, the F.D.A. and the Agriculture Department issued a joint warning to consumers saying that melamine has found its way into hog and chicken feed, encouraging producers to destroy the animals, even though there is no clear evidence that consuming meat from the animals is a danger to human health.



American regulators are now under growing pressure to ensure the safety of human and pet food and to get to the bottom of the melamine scare.



But what began as a pet food recall on March 16 involving two factories working for a single pet food maker, Menu Foods, has now expanded to include some of America’s leading pet food brands and over 60 million pet food packages.



The two Chinese companies named by American regulators last month have said little publicly since the recall. Both companies are based in eastern China, near one of the country’s biggest wheat-growing regions and also one of the centers of melamine production.



Melamine is an industrial chemical that animal feed producers here say has been intentionally mixed into feed to trick farmers into thinking they are buying higher protein meal, even though the chemical has no nutritional value.



A similar practice once took place in the United States and in China involving a related compound called urea, but that compound is now more widely tested for and is banned from certain feeds in the United States.



“This was standard stuff after World War II, when animal feed was adulterated with urea,” said Marion Nestle, a professor of nutrition, food and public health at New York University. “This is simple greed. It’s like they’re adding water to the wheat gluten.”



The Chinese government has told American regulators that Xuzhou is not a manufacturer of wheat gluten but purchased its products from 25 manufacturers.



ChemNutra, the Las Vegas pet food supplier that bought the wheat gluten from Xuzhou and then resold it to pet food makers in North America, also said it was led to believe Xuzhou was the manufacturer of the product.



But ChemNutra officials also say that they received the shipments of wheat gluten through a third party, a company called Suzhou Textiles Silk Light and Industrial Products.



A spokesman for Suzhou Textiles denied that the company exported any wheat gluten to the United States



The other supplier of contaminated protein named by regulators is Binzhou Futian Biology Technology, which says that it supplies soy, corn and other proteins and has strong sales in the United States, Europe and Southeast Asia. The company also declined to comment.



The Chinese government said last week that it was unlikely melamine could have harmed so many pets in the United States. But on Friday, China banned melamine from use in any vegetable protein for export or for use in the domestic food market.



The F.D.A. says that it has received reports that more than 4,000 cats and dogs died as a result of eating pet food that may have been laced with melamine.



Scientists are now struggling to determine why melamine, a chemical that is not believed to be toxic, may have turned poisonous.



Some scientists theorize that melamine mixed with other melaminelike compounds, like cyanuric acid, created a poisonous substance.



And that possibility may be all the more likely because many animal feed producers in China are not using pure melamine but impure melamine scrap that is sold more cheaply as the waste product after melamine is produced by chemical and fertilizer factories here.



“It’s possible the other stuff they were left with was the bottom-of-the-barrel stuff, leftover melamine and possibly cyanuric acid,” said Richard Goldstein, an assistant professor at the Cornell University College of Veterinary Medicine. “I think it’s this melamine with other compounds that is toxic.”