ZHANGQIU, China, April 28 — As American food safety regulators head to China to investigate how a chemical made from coal found its way into pet food that killed dogs and cats in the United States, workers in this heavily polluted northern city openly admit that the substance is routinely added to animal feed as a fake protein.

For years, producers of animal feed all over China have secretly supplemented their feed with the substance, called melamine, a cheap additive that looks like protein in tests, even though it does not provide any nutritional benefits, according to melamine scrap traders and agricultural workers here.

“Many companies buy melamine scrap to make animal feed, such as fish feed,” said Ji Denghui, general manager of the Fujian Sanming Dinghui Chemical Company, which sells melamine. “I don’t know if there’s a regulation on it. Probably not. No law or regulation says ‘don’t do it,’ so everyone’s doing it. The laws in China are like that, aren’t they? If there’s no accident, there won’t be any regulation.”

The Shandong Mingshui Great Chemical Company makes a chemical called melamine and sometimes sells melamine scrap to other producers who use it to make animal feed.

Melamine is at the center of a recall of 60 million packages of pet food, after the chemical was found in wheat gluten linked this month to the deaths of at least 16 pets and the illness of possibly thousands of pets in the United States.

No one knows exactly how melamine (which is not believed to be particularly toxic) became so fatal in pet food, but its presence in any form of American food is illegal.

The link to China has set off concerns among critics of the Food and Drug Administration that ingredients in pet food as well as human food, which are increasingly coming from abroad, are not being adequately screened.

“They have fewer people inspecting product at the ports than ever before,” says Caroline Smith DeWaal, the director of food safety for the Center for Science in the Public Interest in Washington. “Until China gets programs in place to verify the safety of their products, they need to be inspected by U.S. inspectors. This open-door policy on food ingredients is an open invitation for an attack on the food supply, either intentional or unintentional.”

Now, with evidence mounting that the tainted wheat gluten came from China, American regulators have been granted permission to visit the region to conduct inspections of food treatment facilities.

The Food and Drug Administration has already banned imports of wheat gluten from China after it received more than 14,000 reports of pets believed to have been sickened by packaged food. And last week, the agency opened a criminal investigation in the case and searched the offices of at least one pet food supplier.

The Department of Agriculture has also stepped in. On Thursday, the agency ordered more than 6,000 hogs to be quarantined or slaughtered after some of the pet food ingredients laced with melamine were accidentally sent to hog farms in eight states, including California.

The pet food case is also putting China’s agricultural exports under greater scrutiny because the country has had a terrible food safety record.

In recent years, for instance, China’s food safety scandals have involved everything from fake baby milk formulas and soy sauce made from human hair to instances where cuttlefish were soaked in calligraphy ink to improve their color and eels were fed contraceptive pills to make them grow long and slim.

For their part, Chinese officials dispute any suggestion that melamine from the country could have killed pets. But regulators here on Friday banned the use of melamine in vegetable proteins made for export or for use in domestic food supplies.

Yet what is clear from visiting this region of northeast China is that for years melamine has been quietly mixed into Chinese animal feed and then sold to unsuspecting farmers as protein-rich pig, poultry and fish feed.

Many animal feed operators here advertise on the Internet, seeking to purchase melamine scrap. The Xuzhou Anying Biologic Technology Development Company, one of the companies that American regulators named as having shipped melamine-tainted wheat gluten to the United States, had posted such a notice on the Internet last March.

Here at the Shandong Mingshui Great Chemical Group factory, huge boiler vats are turning coal into melamine, which is then used to create plastics and fertilizer.

But the leftover melamine scrap, golf ball-size chunks of white rock, is sometimes being sold to local agricultural entrepreneurs, who say they mix a powdered form of the scrap into animal feed to deceive those who raise animals into thinking they are buying feed that is high in protein.

“It just saves money if you add melamine scrap,” said the manager of an animal feed factory here.

Last Friday here in Zhangqiu, a fast-growing industrial city southeast of Beijing, two animal feed producers explained in great detail how they purchase low-grade wheat, corn, soybean or other proteins and then mix in small portions of nitrogen-rich melamine scrap, whose chemical properties help the feed register an inflated protein level.

Melamine is the new scam of choice, they say, because urea — another nitrogen-rich chemical — is illegal for use in pig and poultry feed and can be easily detected in China as well as in the United States.

“People use melamine scrap to boost nitrogen levels for the tests,” said the manager of the animal feed factory. “If you add it in small quantities, it won’t hurt the animals.”

The manager, who works at a small animal feed operation here that consists of a handful of storage and mixing areas, said he has mixed melamine scrap into animal feed for years.

He said he was not currently using melamine. But he then pulled out a plastic bag containing what he said was melamine powder and said he could dye it any color to match the right feed stock.

He said that melamine used in pet food would probably not be harmful. “Pets are not like pigs or chickens,” he said casually, explaining that they can afford to eat less protein. “They don’t need to grow fast.”

The resulting melamine-tainted feed would be weak in protein, he acknowledged, which means the feed is less nutritious.

But, by using the melamine additive, the feed seller makes a heftier profit because melamine scrap is much cheaper than soy, wheat or corn protein.

“It’s true you can make a lot more profit by putting melamine in,” said another animal feed seller here in Zhangqiu. “Melamine will cost you about $1.20 for each protein count per ton whereas real protein costs you about $6, so you can see the difference.”

Feed producers who use melamine here say the tainted feed is often shipped to feed mills in the Yangtze River Delta, near Shanghai, or down to Guangdong Province, near Hong Kong. They also said they knew that some melamine-laced feed had been exported to other parts of Asia, including South Korea, North Korea, Indonesia and Thailand.

Evidence is mounting that Chinese protein exports have been tainted with melamine and that its use in agricultural regions like this one is widespread. But the government has issued no recall of any food or feed product here in China.

Indeed, few people outside the agriculture business know about the use of melamine scrap. The Chinese news media — which is strictly censored — has not reported much about the country’s ties to the pet food recall in the United States. And few in agriculture here see any harm in using melamine in small doses; they simply see it as cheating a little on protein, not harming animals or pets.

As for the sale of melamine scrap, it is increasingly popular as a fake ingredient in feed, traders and workers here say.

At the Hebei Haixing Insect Net Factory in nearby Hebei Province, which makes animal feed, a manager named Guo Qingyin said: “In the past melamine scrap was free, but the price has been going up in the past few years. Consumption of melamine scrap is probably bigger than that of urea in the animal feed industry now.”

And so melamine producers like the ones here in Zhangqiu are busy.

A man named Jing, who works in the sales department at the Shandong Mingshui Great Chemical Group factory here, said on Friday that prices have been rising, but he said that he had no idea how the company’s melamine scrap is used.

“We have an auction for melamine scrap every three months,” he said. “I haven’t heard of it being added to animal feed. It’s not for animal feed.”

David Barboza reported from Zhangqiu and Alexei Barrionuevo reported from Chicago. Rujun Shen also contributed reporting from Zhangqiu.