The exporter of a contaminated pet food ingredient blamed for the deaths of dogs and cats in the United States may have avoided Chinese export inspections by labeling it a nonfood product, a U.S. government report says.

The company, Xuzhou Anying Biologic Technology Development Co., was not the original producer of the tainted wheat gluten, but may have purchased it from up to 25 different suppliers, the U.S. Food and Drug Administration said in a statement.

The identities of those suppliers remain a mystery and all calls to listed numbers for Xuzhou Anying on Friday rang unanswered. Investigators are also looking into the origins of a second contaminated food additive imported from China, rice protein concentrate.

The New York Times reported that Xuzhou Anying's manager, Mao Lijun, had been detained by Chinese authorities, although the paper gave no details about possible charges against him.

Calls to police and government offices in the city of Xuzhou, in eastern China's Jiangsu where the company is based, rang unanswered on Friday, which was a public holiday.

The scandal concerns the use of the mildly toxic chemical melamine as an additive to animal feed, a practice believed to be common in China, where scandals over contaminated or unsafe food are routine.

Adding the chemical to food is illegal under American law, and while no laws govern the use of melamine in China, the government last week said it was banning its application in food products.

A wave of animal deaths in the U.S. in March was blamed on melamine contamination, prompting one of the biggest recalls of pet food in American history _ more than 100 brands. The recall has since been expanded to include pet food products in Canada and Europe.

The FDA on Thursday said U.S. government inspectors are checking food makers who use protein concentrates to ensure none of their products were contaminated with melamine.

There is no evidence that any of the two contaminated batches of wheat gluten and rice protein from China ended up as an ingredient in human food, "but it's prudent to look," said David Acheson, assistant FDA commissioner for food protection.

Although it has no nutritional value, melamine is high in nitrogen, making wheat gluten and other vegetable products to which it is added appear to have more protein. That allows it to be sold at a premium to farmers and those who use wheat gluten and other additives to make pet food.

The chemical, normally used to make plastics and fertilizers, is not considered a direct health risk to humans. However, scientists say they have too little data to assess how it might react with other chemicals, raising concerns about its introduction into the human food chain through the consumption of meat from animals raised on melamine-spiked feed.

"According to the Chinese government, Xuzhou Anying did not declare the contaminated wheat gluten it shipped to the United States as a raw material for feed or food," the U.S. FDA report said.

"Rather, according to the Chinese government, it was declared to them as nonfood product, meaning that it was not subject to mandatory inspection by the Chinese government," the report said.

China's Foreign Ministry said in a statement last week that the contaminated vegetable protein managed to get past customs without inspection because it had not been declared for use in pet food.

The FDA report said that as of April 26, the FDA had received reports of 1,950 deaths of cats and 2,200 deaths of dogs related to the complaint. Earlier, the administration said it had confirmed only about a dozen pet deaths due to kidney failure caused by melamine ingestion.

The outbreak has drawn wide concern in the U.S., where such pets are often considered members of the family. Bereaved pet owners have sued the firms involved, and FDA investigators have raided the offices of Menu Foods, a maker of pet food, and ChemNutra Inc., which supplied the wheat gluten, according to the companies.

FDA inspectors have been sent to China to investigate the contamination.

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Associated Press writer Andrew Bridges in Washington, D.C., contributed to this story.

(This version CORRECTS that melamine causes kidney failure, not liver damage.)