Three government agencies — the Agriculture Department, the Food and Drug Administration and the Environmental Protection Agency — are overseeing a risk assessment to determine whether the chickens would pose a threat to human health if eaten, USDA spokesman Keith Williams said. The assessment may be completed as early as Monday.

The 20 million chickens represent a tiny fraction of the 9 billion chickens raised each year in the United States. Meat from the birds can't go into commercial use without the USDA's inspection seal, which is being withheld until the risk assessment is completed, Williams said.

Which states have chicken producers affected by the hold will be disclosed later, Williams said.

Investigators have found that about 5 percent of feed used at some smaller chicken production operations contains the chemical melamine, Williams said. Larger manufacturers, because they usually use special feed for the chickens they raise or contract for raising, are unlikely to have exposed their animals to large amounts of the tainted pet products, he said.

As of Friday, no melamine had been detected in the feed used by larger manufacturers, Williams said. However, because investigators know some of the tainted pet food was used in the chicken feed, officials placed a hold on the birds, he said.