Responding to the news, hog and pork producer Smithfield Foods said it is unlikely it fed contaminated feed to its hogs but it is checking all of its feed suppliers.
"We have no reason to believe at this point that we have been exposed to any of these ingredients," said Don Butler, spokesman for Murphy-Brown, the hog production unit of Smithfield Foods.
The FSIS was trying to determine whether the hog farms in the states other than California actually fed the material to their animals, spokesman Steven Cohen said in a statement. Hogs that were confirmed to have eaten the tainted food were processed at a federally inspected facility in California, Cohen said.
"All of that meat is under control at the facility," he said. "It is important to keep in mind this is a small number of farms that may have received this feed."
However, the Food and Drug Administration said the urine of some hogs tested positive for the chemical, melamine, in North Carolina and South Carolina as well as California.
"At this point, I don't have a definitive answer other than to say that the issue is being addressed," Stephen Sundlof, the FDA's chief veterinarian, told reporters when asked if any of the hogs had entered the human food supply. A poultry farm also may be involved, he added.
The FDA also said it planned to begin testing a wide variety of vegetable proteins at firms that imported the ingredients to make everything from pizza dough to infant formula, and protein shakes to energy bars. The ingredient list includes wheat gluten, corn gluten, corn meal, soy protein and rice bran.
Pet food companies have recalled more than 100 brands of cat and dog food since the first reports of animal deaths a little over a month ago.
Investigators have found melamine in at least two imported Chinese vegetable proteins used to make pet foods. The chemical possibly was used to skew analysesthat measured the protein content of the ingredients, wheat gluten and rice protein concentrate.
There were no direct shipments of either of the two ingredients to firms that make food for humans or for animals used as food, said Michael Rogers, who directs field investigations for the FDA.
A second, related chemical called cyanuric acid also has been found to contaminate rice protein concentrate samples, Sundlof said.
The analyses the FDA plans to begin later this week will look at producers of both food for humans and animal feed, said Dr. David Acheson, the chief medical officer within the agency's Center for Food Safety and Applied Nutrition.
Acheson stressed that there was no evidence any of the other vegetable proteins had been contaminated, but that the FDA wanted to "get ahead of the curve" and raise awareness among manufacturers.
FDA officials said the hogs were fed salvaged pet food made with tainted rice protein concentrate. The food was given to the animals prior to the products' recalls, Rogers said. Adulterated food cannot be legally fed to either humans or animals, Sundlof said.
Meanwhile, the FDA is sampling for melamine and related compounds in all wheat gluten, rice protein and corn gluten coming into the United States from China. Also Tuesday, the FDA said another pet food company, SmartPak, had recalled products made with tainted rice protein concentrate. The company said the recall covered a single production run of its LiveSmart Weight Management Chicken and Brown Rice Dog Food.