By Susan Heavey Wed Apr 25, 8:13 AM ET

WASHINGTON (Reuters) - Health officials are now looking at whether humans may have consumed food containing a chemical linked to a recall of pet foods and livestock feed, the U.S. Food and Drug Administration said on Tuesday.

FDA officials said they would inspect imports of six grain products used in foods ranging from bread to baby formula for traces of melamine, a chemical thought to have killed and sickened cats and dogs.

The California Agriculture Department said separately it was trying to contact 50 people who bought pork that may have come from pigs fed food containing melamine. The state's health department recommended humans not consume the meat, but said any health risk was minimal.

Melamine, a chemical used in plastics and fertilizer, has already been found in wheat gluten and rice protein imported from China for use in some pet foods, triggering a recall of more than 100 brands.

The FDA named the six grain products to be inspected as wheat gluten, corn gluten, corn meal, soy protein, rice bran and rice protein.

"We're going to target firms that we know are receiving imported products," said David Acheson, chief medical officer of the FDA's Center for Food Safety and Applied Nutrition in a conference call with reporters. "The goal is obviously to sample as much as we can."

There is little research on melamine's effect on humans, according to World Health Organization, but the chemical has been studied in animals for its risk of kidney problems and cancer. The WHO does not classify the chemical as a carcinogen for people.

Some tainted material was used for hog feed before the contamination was found, and officials said on Tuesday thousands of pigs might be affected on farms in North and South Carolina, California, New York, Utah and possibly Ohio.

The FDA is working with the U.S. Department of Agriculture and several states to investigate the now-quarantined farms and whether hogs on those farms were slaughtered for human food.

"Some of the hog operations were fairly sizable," said Stephen Sundlof, director of the FDA's Center for Veterinary Medicine. But USDA spokesman Steve Cohen said the feed was sold to smaller and independent hog farms.

A poultry farm in Missouri also may have received tainted feed, officials added.

Still, the FDA has no intention of banning imports of wheat gluten, rice protein or similar products from China.

"We believe the safety net is in place to make sure that no additional products are going to get into the commerce of the United States," said David Elder, director of FDA's enforcement office.

Melamine was first found in March in wheat gluten used for some pet foods. Menu Foods, Procter & Gamble Co., Colgate-Palmolive Co., Nestle SA and Del Monte Foods Co. have recalled pet products made with the gluten.

More recently, rice protein tainted with melamine was also shipped to at least five pet food manufacturers by a supplier that imported it from China, the FDA has said.

On Monday, two U.S. lawmakers said a second company likely imported rice protein from China that was contaminated with Melamine. FDA officials on Tuesday would not say whether there was a second importer.

(Additional reporting by Charles Abbott)