XUZHOU, China - Behind an unmarked gate in this booming city well north of Shanghai lies a large building at the heart of an investigation over tainted pet food that has killed at least 16 cats and dogs in the United States, sickened 12,000 and prompted a nationwide recall.

This is the property of the Xuzhou Anying Biologic Technology Development Co., a small agricultural products business investigators have identified as the source of contaminated wheat gluten that was shipped to a major pet food supplier in the United States.

Some American regulators suspect there was deliberate mixing of substances. They are looking into the possibility that melamine, the chemical linked to the pets' deaths, was mixed into the wheat gluten in China as a way to bolster the protein content, according to a person who was briefed on the investigation.

Though American and Chinese regulators are searching for answers, local residents and workers are unwittingly providing clues about how the pet food supply may have become contaminated.

The case is also exposing some of the enormous challenges confronting the global marketplace as China becomes a worldwide supplier of agricultural products.

There are strong indications that Xuzhou Anying, a company whose main office seems to consist of just two rooms and an adjoining warehouse here, possessed substantial supplies of melamine and even sought to buy quantities of it over the Internet.

If melamine was intentionally blended into the wheat gluten, the findings could become a vast setback for agricultural trade between the United States and China, a country known for lax food-safety regulations.

Stephen Sundlof, director of the Center for Veterinary Medicine at the Food and Drug Administration, said at a news conference last week that the agency had found unusually high concentrations of melamine in some batches of wheat gluten, as much as 6.6 percent.

Xuzhou Anying, though, has tried to distance itself from the pet food recall in the United States, saying it does not manufacture or export wheat gluten and acts only as a middleman trading in agricultural goods and chemicals.

In a telephone interview last week, the company's manager, Mao Lijun, said he had no idea how wheat gluten with his company's label ended up in the United States or how melamine, a chemical commonly used to make plastics, fertilizer and fire retardant, was mixed into a product that was eventually shipped there.

Though some American scientists still question whether melamine is toxic enough to kill pets, the chemical is not approved for use in human or pet food in the United States. The FDA says it may have led to kidney failure in some pets.

Here in Xuzhou, a metropolitan region of about 1.6 million, Mao turned away visitors to his office, declaring that he had nothing more to say on the matter.

But there are indications that Xuzhou Anying has manufacturing facilities in this area and also had access to melamine, which is sometimes used as a fertilizer in Asia. For instance, in recent months Xuzhou Anying has posted several requests on Web trading sites seeking to purchase large quantities of melamine..

Chinese regulators say they are now carrying out a nationwide inspection of wheat gluten supplies. American regulators have banned all wheat gluten from China, but there has been no recall so far in China of wheat gluten made by Xuzhou Anying, though the company's wheat gluten could be used to make bread, bakery and other food items."