SHANGHAI, China - The exporter of a contaminated pet-food ingredient blamed for the deaths of what officials now say are thousands 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. Calls to listed numbers for Xuzhou Anying as well as to government offices in the eastern China city of Xuzhou, where the company is based, rang unanswered yesterday, which was a public holiday.

Investigators are also looking into the origins of a second contaminated food additive imported from China, rice protein concentrate.

The New York Times reported yesterday that Xuzhou Anying's manager, Mao Lijun, had been detained by Chinese authorities, although possible charges against him were not detailed.

The issue 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 although 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 deaths of pets in the United States 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.

As the recall expanded over several weeks, the administration said it had confirmed only about a dozen pet deaths due to kidney failure caused by melamine ingestion.

A week ago, the FDA said it had received reports of 1,950 deaths of cats and 2,200 deaths of dogs related to the complaint.

Then yesterday, the Los Angeles Times reported that federal officials said consumers had called in the deaths of as many as 8,500 dogs and cats as a result of tainted pet food. About half the calls to the FDA hotline over the last two months were from owners of deceased cats and dogs, the paper said.

Officials said the agency had not confirmed those reports, but added that the numbers of allegations likely will increase as it catches up with a backlog of calls.

On Thursday, meanwhile, the FDA said U.S. government inspectors were checking food makers who use protein concentrates, to ensure that 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.