Canadian researchers said Friday they've made a chemical discovery that may explain how contaminated pet food blocked animals' kidney function, sickening hundred of dogs and catsin Canada and the United States.

'We knew these two compounds had been implicated, but because neither seemed sufficiently toxic on its own, it was unclear how they might have been involved.'—Perry Martos, University of Guelph

University of Guelph researcher Perry Martos said animals likely developed crystals in their kidneys because of a reaction between cyanuric acid and melamine— a chemical used to make plastics and fertilizer, and found in the recalled pet food.

"We knew these two compounds had been implicated, but because neither seemed sufficiently toxic on its own, it was unclear how they might have been involved," Martos said in a statement.

Martos said tests on the affected animals have shown the presence of crystal-like substances in the animals' kidneys and urine samples.

More pet food withdrawn

Meanwhile, Diamond Pet Foods said Friday it was pulling three of its products from the marketplace because they are believed to contain tainted rice protein concentrate.

The recall applies to Diamond Lamb & Rice Formula for Dogs (13 oz. cans), Chicken Soup for the Pet Lover's Soul Kitten Formula (5.5 oz. cans) and Chicken Soup for the Pet Lover's Soul Puppy Formula (13 oz. cans). The products were available in Canada.

The Meta, Mo.-based company said it issued the recall after the U.S. Food and Drug Administration confirmed the rice protein concentrate used by the manufacturer was contaminated with melamine.

ChemNutra offices searched

AU.S. supplier of wheat gluten also believed to be tainted with melamine, ChemNutra, saidfederal agentssearched its Las Vegas offices. The company said it imported the gluten from a supplier in China.

"We have co-operated and complied fully with FDA investigators both prior to and since being served with today's search warrant, and will continue to do so," Steve Miller, chief executive officer of ChemNutra Inc., said in a statement Friday.

"We keep very good records, which has made it relatively easy for the investigators to retrieve what they needed."

Menu Foods Midwest, an affiliate of Ontario-based Menu Foods Ltd., filed a lawsuit against ChemNutra this week, seeking payment for the costs of its extensive recall of 60 million cans of pet food, along with damages.

ChemNutra countered that Menu Foods knew of a problem related to the pet food but waited several weeks before acting. ChemNutra officials also said they were not Menu Food's sole wheat gluten supplier.

More than 50 lawsuits have been filed against Menu Foods.

The FDA traced the problem to melamine after pet owners grew concerned about apparent kidney failure in their cats and dogs. There have been 16 confirmed cases of animals dying in the U.S. after eating Menu Foods products.

With files from the Associated Press