Hogs at a farm in California ate the contaminated products, and officials were trying to determine whether hogs in Utah, New York, North Carolina, South Carolina and Ohio may have eaten the tainted food. Hogs at some of the farms - it wasn't immediately clear which - have been quarantined.
A spokesman for the Food Safety and Inspection Service, Steven Cohen, said in a statement that the group was trying to determine whether the hog farms in the states other than California actually fed the material to their animals.
Larry Lewis, spokesman for the Utah Department of Agriculture and Food, said his agency has no evidence that hogs at state operations ate the tainted food or that any had been quarantined, and he has asked his federal counterparts to provide more information. Utah is ranked 16th in the nation in the production of hogs.
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.''
The urine of some hogs tested positive for the chemical, melamine, the Food and Drug Administration said.
''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 various items including pizza dough, infant formula, protein shakes and 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 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.