XUZHOU, China, April 10 — 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 Company, a small agricultural products business that 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 with a main office that 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.

Mixing melamine and wheat gluten is an unlikely practice here, according to local industry participants. Nonetheless, the company’s wheat gluten, tainted with melamine, ended up in millions of packages sent to the United States and Canada, leading to one of the biggest pet food recalls ever.

ChemNutra, the Las Vegas-based company that acknowledges it imported the wheat gluten from Xuzhou for sale to pet food producers in North America, says Xuzhou Anying provided chemical analyses that showed no impurities or contamination in the packages of wheat gluten.

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 F.D.A. says it may have led to kidney failure in some animals.

The question that regulators, agriculture experts, and food producers and distributors may now be asking is whether other substances added to food imports can broadly contaminate the American food supply. The F.D.A. has said none of the contaminated wheat gluten leaked into human food.

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

But there are indications that Xu- zhou 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.

In a March 29 posting on a site operated by Sohu.net, a big Chinese company, officials of Xuzhou Anying wrote, “Our company buys large quantities of melamine scrap all year around.” There were also postings on several other trading sites like ChemAbc.net.

A truck driver parked across the street from the company’s main office here said that Xuzhou Anying did operate manufacturing facilities and that he carried goods for the company.

“Yes, they have a factory that makes wheat gluten,” said the man, who did not give his name and then telephoned the manager of Xuzhou Anying to check whether he could take visitors to the factory.

On Tuesday, a reporter visited one of the facilities the truck driver identified in the village of Wangdian, about 10 miles south of company headquarters, but the gate to the building was padlocked.

Storage sacks that appeared to hold grain or agricultural supplies were stacked outside the site in a vast wheat- and garlic-growing region here in Jiangsu Province.

They used to have their headquarters right over there,” said Chen Wei, a technology director at Nanjing Shibide Biologic Technology, an animal-feed company next door. “They’re pretty well known for their products.”

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 domestic recall so far of gluten produced by Xuzhou Anying; the company’s wheat gluten can be used to make bread, baked goods and other food.

Li Jundang, manager of Shandong Binzhou Tianjian Biotechnology, a wheat gluten producer in the city of Binzhou, about 200 miles north of here, said, “We never heard the news of tainted pet food.” Another gluten exporter, Shandong Rongchang, also said it was unaware of any problems with Chinese wheat gluten.

Nor, it seems, have journalists in Xuzhou, who work under state censorship. “I didn’t know this news about Xuzhou Anying,” said Li Ning, news director at The City Morning Post, a daily newspaper here. “And even if we had heard about the news, we wouldn’t be able to report on it because it’s negative news.”

Most experts on wheat gluten in the region said they had never heard of mixing it and melamine.

“If you add chemicals into the wheat gluten, it is no longer called wheat gluten protein,” says Jiang Shaotong, a professor of food engineering at Hefei University of Technology in nearby Anhui Province. “I can’t think of any reason why melamine is needed in the production process.”

Chinese customs officials do inspect or sample products planned for export, but those inspections are not thought to be stringent enough to detect the presence of every chemical or impurity.

Asked about the investigation, a Chinese official working for the inspection and quarantine bureau declined to comment.

But lax food-safety regulation and standards are a problem; food producers sometimes dye meats to make them look fresher and even sell fake milk powder for babies.

This week, the Chinese government reported that an elderly woman died and 202 people were sickened at a hospital north of here after they consumed a breakfast cereal that turned out to be laced with rat poison.