WASHINGTON — Federal officials on Friday placed a hold on 20 million chickens raised for market in several states because their feed was mixed with pet food containing an industrial chemical.

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 announced later, Williams said.