The chemically adulterated Chinese protein products that wound up in U.S. pet food -- killing at least 16 dogs and cats and sickening thousands of others -- have apparently entered the human food chain via meat.

The U.S. Department of Agriculture and Food and Drug Administration this week announced that byproducts from pet food manufactured with contaminated Chinese gluten have been used in chicken feed on some farms in Indiana. The investigation so far has found that about 30 broiler poultry farms received contaminated feed in early February and fed it to poultry within days of receiving it. All of the broilers believed to have been fed the contaminated product have since been processed, the agencies say -- and presumably eaten.

The agencies claim the likelihood of harm to humans eating contaminated chicken is low because the contamination would have been diluted. They say they will not issue recalls of the processed chicken without evidence of harm to people.

The imported products in question are believed to have been intentionally adulterated with melamine, a chemical used in producing plastics, and a related compound, cyanuric acid. China has since banned the addition of melamine to such foodstuffs, a practice reportedly done to create the appearance of a higher protein content, which in turn demands higher prices. While scientific research indicates melamine is not acutely toxic to humans, according to the USDA and FDA, there's no data on the effects of combining it with compounds like cyanuric acid.

The first inkling that the poisoned protein products could have made their way into the human food supply came last week, when the USDA announced that North Carolina and South Carolina were among eight states where hog farmers had purchased chemically adulterated animal feed from China. The USDA has said it would compensate pork producers who euthanize animals that consumed the adulterated feed.

After the House Committee on Energy and Commerce held a hearing last week on the safety of the nation's food supply, the FDA opened a criminal investigation into the contaminated Chinese products and ordered the immediate embargo of all vegetable protein from China, whether intended for human or animal food. In addition, the committee dispatched investigators to the West Coast to "examine the extent and depth of the FDA's commitment to inspection of these products from China," according to a statement from Chair John Dingell (D-Mich.).

Rep. Bart Stupak (D-Mich.) emphasized that this is just the beginning of the committee's investigation: "The FDA should have taken this action weeks ago," he said. "Our committee will continue to monitor this situation and will continue to pressure the FDA to properly address the food safety problems that have plagued our country in recent months."