Story Highlights• Researchers have found melamine and cyanuric acid in pet food
• Scientists: Together, the two chemicals can form fatal crystals in kidneys
• FDA: 17,000 consumer complaints, more than 4,000 pet deaths
• FDA will begin to stop importation of some products for human food use
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(CNN) -- Scientists from Canada and the United States say they have new evidence for why dogs and cats died after eating contaminated pet food.
Owners of more than 4,000 pets have complained to the federal Food and Drug Administration that their animals died after eating food that was later recalled.
Inspectors found melamine in the tainted products, but not at levels that would normally kill. But researchers now say that it may have mixed with another compound -- cyanuric acid -- to produce crystals that may have been deadly.
"What we've done is experiments that show if you take cat urine and you add melamine to it and cyanuric acid, the crystals will form in the cat urine in a test tube as we're watching them, so it happens within a matter of hours," said Alan Wildeman, vice president of Canada's University of Guelph, which is renowned for its veterinary research center.
The crystals are suspected of contributing to kidney failure in pets.
"I think we've identified what we feel is an important and likely underlying positive agent of why the animals are getting sick," Wildeman said.
Crystals blocked the tubes leading from the kidneys to the bladder in one cat operated on recently at the Bergh Memorial Animal Hospital inside the ASPCA's New York headquarters, according to Dr. Louise Murray, the group's director of medicine.
"The cat's kidneys were completely obstructed, and when we went to surgery to relieve the obstruction, there was no normal stone. Instead, the ureters were completely full of these melamine-type crystals," she said.
The FDA has confirmed that rice protein and wheat gluten imported from China were contaminated with both melamine and cyanuric acid -- a combination of which is "a potential source of concern in relation to human and animal health," according to David Elder of the agency's Office of Regulatory Affairs.
Dr. Dan McChesney, a veterinary expert with the FDA, said melamine was "surely associated" with the pet deaths and further research was being carried out to see what other factors may have been involved.
The FDA said Friday it had received more than 17,000 consumer complaints about the tainted pet food, including the deaths of 1,950 cats and 2,200 dogs.
The FDA has officially tallied 16 animal deaths related to the pet food recall, which spread to include more than 60 million packages of nearly 100 brands of pet food.
Human food supply may be at risk
The FDA will begin to prevent the importation of vegetable protein products from China for human food use that may contain melamine.
"We see the pet food recall as a warning sign for the government that they need to do more to protect the food supply," said Caroline Smith DeWaal of the Center for Science in the Public Interest. "It could easily happen to an ingredient used in human food as well."
The FDA says it has collected 750 samples of wheat gluten and products made with wheat gluten and found that 330 -- about 44 percent -- tested positive for melamine or melamine-related compounds.
It also found that out of 85 samples of rice protein products, 27 were positive for melamine. All of the positive samples were imported from China.
The agency has already announced that 6,000 hogs in several states that may have eaten tainted feed should be destroyed, and not put into the human food chain.
CNN's Joe Johns and Daniel Ruetenik contributed to this report.
Miami, Florida, vet Michael Fusco checks Chloe, whose owner brought her in fearing she may have eaten tainted food, in March.
Sources: OSHA, CDC