The front of the Xuzhou Anying Biologic Technology Development Co, near Xuzhou, China, is seen on April 11. The company is one of two Chinese firms at the center of the huge recall of pet food that has killed or sickened thousands of animals. It shipped hundreds of tonnes of wheat gluten labeled as nonfood products this year through a third-party Chinese textile company. The company avoided inspections partly because it did not correctly disclose its shipping contents to Chinese export authorities, US regulators have said.

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

The Xuzhou Anying Biologic Technology Development Co, 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 635 tonnes of wheat gluten labeled as nonfood products this year through a third-party Chinese textile company.

By listing the goods as nonfood items, the firm's shipments were not subject to mandatory inspection by the Chinese government.

Though a possible violation of Beijing's 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 US Food and Drug Administration (FDA), are just the latest clues that Chinese feed suppliers may have been intentionally disguising the contents of their goods.

Several FDA 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.

US regulators admit that six weeks after one of the biggest pet food recalls in US history, they still do not know exactly 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 US, regulators suspect the companies may not have been manufacturing the feed, but buying it from dozens of other feed manufacturers in China.