A Chinese company accused of selling contaminated wheat gluten to pet food suppliers in the United States failed to disclose to China's export authorities that it was shipping food or feed to the United States, thereby avoiding having its goods inspected, according to U.S. regulators.

Xuzhou Anying Biologic Technology Development, one of two Chinese companies at the center of the massive pet food recall after thousands of animals were killed and sickened, had shipped more than 700 tons of wheat gluten labeled as "nonfood" products earlier this year through a third party, a Chinese textile company.

The "nonfood" designation meant the company's shipments were not subject to mandatory inspection by the Chinese government.

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

FDA officials are now visiting China, seeking more information about how and why an industrial chemical used in plastics and as fertilizer got mixed into pet food ingredients.

The pet food recall is threatening to turn into a major trade issue because of mounting worries in the U.S. Congress about the safety of China's agricultural exports to the United States, the ability of American regulators to protect the country's food supply and the slow pace of efforts by the Chinese government to aid the investigation.

American regulators admit that six weeks after one of the biggest pet food recalls in U.S. 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, regulators suspect the companies may not have been making the feed but buying it from other feed manufacturers in China. Those producers, regulators believe, may have intentionally mixed melamine into the feed to artificially inflate 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 FDA said. "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," the FDA said.

Worried that the contaminant could continue to enter the United States and also seep into the human food supply through 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 country from China, including corn gluten and soy protein.

Last week, the FDA and the U.S. Department of Agriculture jointly warned consumers that melamine had found its way into hog and chicken feed and encouraged producers to destroy the animals, even though there was no clear evidence that consuming meat from the animals was 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.

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 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 cheat farmers into thinking they are buying higher protein meal, even though the chemical has no nutritional value. A similar practice 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 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."

Contacted a few weeks ago, Xuzhou Anying officials denied having any knowledge of how melamine got into one of their packages and insisted that they never exported any wheat gluten to the United States.

But Xuzhou Anying advertised on the Internet as recently as late March seeking to buy large quantities of melamine scrap. "We urgently need a lot of melamine scrap," Xuzhou said in a posting March 21 on a trading Web site called EC21, leaving a cellphone number.

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

ChemNutra, the Las Vegas pet food supplier that bought the wheat gluten from Xuzhou and then resold it to pet food makers in the United States, 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 & Industrial Products. A spokesman for Suzhou Textiles denied that the company exported 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 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 FDA 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 melamine-like compounds, such as cyanuric acid, to create a poisonous substance.

That possibility may be all the more likely because many animal feed producers in China are not using pure melamine but much cheaper melamine scrap, sold as a waste product from melamine production.