Hogs at a farm in California ate the contaminated products, according to the Food Safety and Inspection Service. Officials were trying to determine whether hogs in New York, North Carolina, South Carolina, Utah and Ohio also may have eaten the tainted food, the FSIS said. Hogs at some of the farms — it wasn't immediately clear which — have been quarantined.
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.
A spokesman for the Utah Department of Agriculture said his office was unaware of any feed problems in Utah.
"We're not aware of any quarantines on hog farms in Utah and we haven't imposed any quarantines," said Larry Lewis.
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.
The FDA said it was unclear whether any people consumed the contaminated pork, CBS News correspondent Nancy Cordes reported.
"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.
Also Tuesday, families victimized by tainted spinach and peanut butteron recent high-profile outbreaks of foodborne illnesses, urging lawmakers to strengthen federal oversight of the nation's food supply.
The FDA 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.
Despite the testing, a former FDA deputy commissioner said the agency is so understaffed it has little capacity to prevent outbreaks, even predictable ones.
"I think basically FDA right now is really not able to protect the safety of the food supply the way people expect," Michael Taylor, the former FDA deputy commissioner for policy, told CBS News.
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 analyses that 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.