The discovery expands the month-long 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 believed to have contaminated rice protein concentrate used to make a variety of Natural Balance Pet Foods products for both dogs and cats, the Food and Drug Administration said Wednesday.
The FDA has there is no evidence so far 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. But the FDA, which is continuing to update its list of pet food recalls, has not yet accounted for all of the imported ingredients.
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.
A lawmaker said Wednesday the Chinese have refused to grant visas to FDA inspectors seeking to visit the plants where the ingredients were made. An FDA spokesman later said the visas were not refused but that the agency had not received the necessary invitation letter to get visas.
"It troubles me greatly the Chinese are making it more difficult to understand what led to this pet food crisis," Sen. Dick Durbin, D-Ill., told The Associated Press after meeting with the FDA commissioner, Dr. Andrew von Eschenbach.
A House committee is holding a food safety hearing Tuesday and is expected to discuss the pet food recall.
Natural Balance said it is recalling all its Venison and Brown Rice canned and bagged dog foods, its Venison and Brown Rice dog treats and its 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 its recalled products contain melamine. Natural Balance believes the source of the contaminant was rice protein concentrate, which the company recently added to the dry venison formulas.
A San Francisco company, Wilbur-Ellis Co., began importing the ingredient in July from a Chinese company, Futian Biology Technology Co. Ltd., according to Wilbur-Ellis president and chief executive John Thacher.
It resold the ingredient to five pet food manufacturers, including Diamond Pet Foods Inc. of Meta, Mo. Diamond manufactured the dry dog and cat foods recalled by Natural Balance, Diamond Pet Foods spokesman Jim Fallon said.
Thacher declined to identify his company's other four customers, except to say two tested the ingredient and found no melamine. Wilbur-Ellis has not heard from the other two, both of whom received limited amounts of the ingredient, Thacher said.
The FDA's tests detected melamine in a rice protein sample; the agency would not disclose the sample's origin.
The source of the melamine remains unclear. It may have contaminated the rice protein through the reuse of dirty bags used to ship the products.
Thacher said an April 4 delivery from Futian Biology included 146 1-ton bags of rice protein concentrate. All were white except for a single pink bag, which was stenciled "melamine."
Wilbur-Ellis isolated the entire shipment at a Portland, Ore. warehouse and sent out samples for testing. The pink bag's contents tested positive for melamine while the two white bags tested were negative, Thacher said.
Futian Biology later told Wilbur-Ellis that a damaged bag was replaced with a clean one, Thacher said. The company then "certified the product was all fine," he added.
The Las Vegas importer of the contaminated Chinese wheat gluten, ChemNutra Inc., that led to the original pet food recall has suggested that spiking a product with melamine can make it to appear to be richer in protein during tests, thus increasing its value.
ChemNutra also imported rice protein concentrate from China, though from another source. Spokesman Steve Stern said the company is testing those shipments.
The recalls began March 16 when Menu Foods recalled 60 million cans of dog and cat food after the deaths of 16 pets, mostly cats, that had eaten its products. The FDA said tests indicated the food was contaminated with melamine, which is used in making plastics and other industrial processes.
Five other companies later recalled pet products also made with wheat gluten tainted by the chemical. The FDA has since blocked Chinese imports of wheat gluten.
Menu Foods continues to add more varieties to its recall list. Menu Foods spokesman Sam Bornstein did not know if the Streetsville, Ontario-based company also used rice protein concentrate as an ingredient in its pet foods, sold under more than 100 different major and store brands.