At least 2.5 million broiler chickens from an Indiana producer were fed pet food scraps contaminated with the chemical melamine and subsequently sold for human consumption, federal health officials reported yesterday.

Hundreds of other producers may have similarly sold an unknown amount of contaminated poultry in recent months, they added, painting a picture of much broader consumption of contaminated feed and food than had previously been acknowledged in the widening pet food scandal.

Officials emphasized that they do not believe the tainted chickens -- or the smaller number of contaminated pigs that were reported to have entered the human food supply -- pose risks to people who ate them.

"We do not believe there is any significant threat of human illness from this," said David Acheson, the Food and Drug Administration's chief medical officer. FDA Commissioner Andrew von Eschenbach named Acheson yesterday the agency's new "food czar" -- officially, assistant commissioner for food protection.

None of the farm animals is known to have become sick from the food, and very little of the contaminant is suspected of having accumulated in their tissue. Thus, no recall of any products that may still be on store shelves or in people's freezers is planned, officials said.

Nonetheless, 100,000 Indiana chickens that ate the melamine-laced food and are still alive have been quarantined and will be destroyed as a precautionary measure -- as will any other animals that turn up as the investigation continues to expand.

The revelations are the latest in a rapidly widening scandal that started out with reports of a few deaths of pets. It has mushroomed into a major debacle that, even if no human injuries emerge, has exposed significant gaps in the nation's food-safety system.

In the first volley of what Hill watchers expect to be a series of proposed fixes, Sen. Richard J. Durbin (D-Ill.) and Rep. Rosa DeLauro (D-Conn.) yesterday introduced legislation that would give the FDA the power to order mandatory recalls of adulterated foods, establish an early warning and notification system for tainted human or pet food, and allow fines for companies that do not promptly report contaminated products.

Meanwhile, the FDA expanded the number of plant-based protein products from China on its "do not import" list, pending the completion of further tests on various kinds of glutens, protein concentrates and other products.

At the center of the problem are pet foods spiked with melamine, a mildly toxic chemical that can make food appear to have more protein than it does. Most of the food went to pets, but scraps were sold in February to the Indiana poultry producer, officials said. The contaminated material may have made up about 5 percent of the chickens' total food supply.

That small fraction, and the fact that people, unlike pets, do not eat the same thing day after day, suggests that consumers who ate contaminated pork or chicken would probably have ingested extremely small doses of melamine, well below the threshold for causing health effects, officials said. Experts conceded, however, that they know little about how the toxin interacts with other compounds in food.

Investigators are tracking streams of the contaminated food through several states.

"Our sense is that the investigation will lead to additional farms where contaminated feed may have been fed to either animals or poultry," said Kenneth Petersen of the Agriculture Department Food Safety and Inspection Service.

Officials said the FDA has received 17,000 reports of pets that owners believe were sickened or killed by contaminated food. About 8,000 reports, roughly half of them involving animals that died, have been formally entered into the FDA's tracking system for further analysis.

U.S. investigators have arrived in China, officials said, but inspections of production facilities there have been hampered by the start yesterday of a week-long national vacation.

"Essentially, all the officials are on holiday," said Walter Batts, part of the FDA's China team, adding that one Chinese official had stayed behind to help.