As the voluntary recall of tainted pet food widens, the U.S. Food and Drug Administration announced yesterday it will begin testing food made with ingredients imported from China and intended for human consumption.

Federal officials will test wheat gluten and five other "protein concentrates" for traces of melamine, the industrial chemical found in pet foods that have sickened and killed dogs and cats.

Federal officials also are investigating hog farms in five states and a chicken farm in one state because those animals may have eaten melamine-tainted pet food.

Tests at U.S. food-producing plants will begin later this week, with federal officials looking at wheat gluten, corn gluten, rice protein concentrate, cornmeal, soy protein and rice bran, FDA officials announced yesterday during a telephone news conference.

Those protein products are used in some pet foods as well as in a wide range of human food products, including bread, pasta, cereal, baby formula, energy bars, protein shakes and "foods often eaten by vegetarians," said Dr. David Acheson, chief medical officer of the FDA's Center for Food Safety and Applied Nutrition.

"We are going to target" U.S. food-producing firms "that we know are getting imported products" from China, he said.

While there is no evidence suggesting that melamine-tainted proteins went into human food, Dr. Acheson said federal officials were being "pro active."

The U.S. plant inspections, he said, are "all about getting out ahead of the curve. ... It's just a smart public health move."

Last week, FDA announced hogs on one small farm in California had eaten tainted pet food. Yesterday, however, officials said farms in five other states -- North Carolina, South Carolina, Utah, New York and possibly Ohio -- will be scrutinized.

"It potentially affects thousands of hogs," said Dr. Stephen F. Sundlof, a veterinarian who is director of the FDA's Center for Veterinary Medicine. "Some of the hog operations are pretty sizeable."

He said officials are checking a Missouri poultry farm to see if chickens ate melamine-tainted feed.

Traces of melamine were found in the urine of hogs in California, North Carolina and South Carolina, officials said. FDA officials are working with officials from the U.S. Department of Agriculture and several states, testing urine and meat of hogs in Utah, New York and Ohio. Quarantines are in effect at some farms where hogs are suspected of eating tainted pet food.

The pet food recall began March 16. More than 100 companies have voluntarily recalled products. Tests done by FDA and other laboratories have found pet foods laced with melamine, which is used to produce plastic utensils in this country and fertilizer in some other countries, including China.

The FDA has received more than 15,000 consumer complaints since the pet food recall began. It has not tallied reports of dogs and cats that have become sick and died from kidney damage after eating tainted pet food.

Most of the melamine was traced to wheat gluten imported from China. In the past two weeks, federal officials have found melamine in rice protein concentrate and corn gluten used to make pet foods.

Yesterday, FDA officials said one more pet food company joined the recall list. SmartPak Canine has recalled one production run -- less than 1,200 pounds -- of LiveSmart Weight Management Chicken and Brown Rice Dog Food.

FDA officials have said they are investigating the possibility that Chinese suppliers intentionally put melamine into wheat gluten to make it appear the pet food was higher in protein that it actually was. U.S. officials want to send food inspectors to Chinese food-processing plants, but have not received permission from the Chinese government.