FDA on pet food:

Federal Food, Drug and Cosmetic Act

The Federal Food, Drug and Cosmetic Act requires that pet foods, like human foods, be pure and wholesome, contain no harmful or deleterious substances, and be truthfully labeled. The Act states, among many other things, that a food may be deemed to be adulterated if it contains poisonous or deleterious substances which may render it injurious to health; if it has been prepared, packed, or held under unsanitary conditions whereby it may have been contaminated with filth or rendered injurious to health; if it contains any part or product of a diseased animal; or if its container is composed of any poisonous or deleterious substance which may render its contents injurious to health.