Testing found that the contaminant, the industrial chemical melamine, was so diluted that any human exposure from the meat or eggs of the animals would be thousands of times lower than the level considered safe, he said.

"We literally found that the dilution is so minute, in fact in some cases you can't even test and find melamine any more in that product," Agriculture Secretary Mike Johanns said during comments to the Organic Trade Association.

Based on the testing, the government lifted a quarantine on nearly all of the 20 million chickens who were held from market last week after the feed contamination was discovered. Those animals now may be slaughtered and enter the food supply.

About 6,000 hogs still were being held pending additional testing. Johanns said he hopes to announce the release of those animals by the weekend. The animals still being held either ate feed that tested positive for contamination or ate feed that no longer was available to test.

USDA spokeswoman Terri Teuber said the government is confident the hogs and remaining chickens are safe to consume, but wanted additional testing before releasing them into the food chain.

Melamine, a chemical found in plastics and pesticides and not approved for use in pet or human in the U.S., contaminated pet food that either sickened or killed an unknown number of dogs and cats.

Scraps left over from the manufacture of that pet food was sold for use in animal feed before the pet food was known to be tainted and recalled from store shelves.

The government initially said about 5 percent of feed used at some smaller chicken production operations had been contaminated. But Monday, Johanns said some larger producers also had been affected. He did not specify the producers.

Since the pet food scraps made up only a small amount of the rations fed to the farm animals, they appear to have been exposed to smaller amounts of melamine than was the case with cats and dogs, officials said.

Even pigs and chickens known to have eaten contaminated feed appear to be healthy, the USDA and FDA said.

Fewer than 500 animals made it to market prior to the quarantine, but Johanns said testing found little reason for concern.

For people who ate large amounts of contaminated pork, chicken and eggs, they likely would be exposed to contamination at levels 18,000 to 30,000 times lower than that considered safe.

Even under the most extreme scenario, the potential human exposure to melamine was well below any level of public health concern, the USDA and FDA said. In that worst-case scenario, government scientists assumed all the solid food a person ate in a day was contaminated with melamine at levels seen in animals fed contaminated feed; that potential exposure was still about 2,500 times lower than the dose considered safe.