The discovery expands the monthlong cascade of recalls to include more brands and varieties of pet foods and treats tainted by the chemical.

"This has exposed that the safety standards for pet foods are not in place in any significant way, and the kind of drumbeat, day after day, of recalls has shaken consumers' confidence in the pet food industry's adherence to food safety standards," said Wayne Pacelle, president and chief executive officer of the Humane Society of the United States.

The chemical, melamine, is thought to have contaminated rice protein concentrate used to make a variety of Natural Balance Pet Foods products for dogs and cats, the Food and Drug Administration said Wednesday.

The FDA has said there is no evidence to suggest any of the rice protein went to companies that make human food, said Michael Rogers, director of the agency's division of field investigations. However, the FDA has not accounted for all of the imported ingredient.

Previously, the chemical was found to contaminate wheat gluten used by at least six other pet food and treat manufacturers.

Both ingredients were imported from China, though by different companies and from different manufacturers.

The FDA on Wednesday began reviewing and sampling all rice protein concentrate imported from China, much as the agency has been doing for wheat gluten, Rogers said.

Natural Balance said it was recalling all of its Venison and Brown Rice canned and bagged dog foods, iVenison and Brown Rice dog treats, and Venison and Green Pea dry cat food.

The recalls now include products made by at least seven companies and sold under more than 100 brands.

The Pacoima, Calif., company said recent laboratory tests showed that its recalled products contain melamine. Natural Balance thinks the source of the contaminant was rice protein concentrate, which the company recently added to the dry venison formulas.