BOSTON -- Officials said they may have found the source of contamination in million of packages of pet food -- rat poison.

NewsCenter 5's Jorge Quiroga reported that the deaths of pets across the country prompted the massive recall of dog and cat food.

Canadian-based Menu Foods recalled 60 million cans and pouches of tainted pet foods sold under about 95 brand names.

Scientists from the New York Department of Agriculture said on Friday that they think they know the source of the contamination.

"Aminopterin, which is a toxic chemical and has been identified as a rodenticide -- although it is not registered in the United States," said Commissioner Patrick Hooker.

ABC News reported that the chemical was put in wheat imported from China that was used by Menu Foods to produce gluten, a filler in wet pet food.

"This is one step in a long process that will lead all of us to know what has happened and how it has happened," Hooker said.

From Boston to California, veterinarians said they are overwhelmed by calls. Fifteen cats and dogs are confirmed dead from kidney failure, but experts said the real number is probably much higher. In the last three weeks, three cats were euthanized at the Angell Animal Medical Center after suffering kidney failure.

"First thing is to try to confirm that they have been exposed to the questionable dog or cat foods by checking certain Web sites and making sure that, indeed, they did have exposure. If that is the case, then absolutely, they should speak to their veterinarian," said Dr. Kiko Bracker of Angell Animal Medical Center.

Danielle DeVito said her two cats, Marvin and Princess, started to show signs of kidney or renal failure after eating foods included in the recall.

"I kept hearing about renal failure, and they do say sometimes at a certain point, it is irreversible," DeVito said.

Her cats were treated and are expected to recover, but DeVito said she now has about $500 in vet bills. ABC News reported Friday that Menu Foods will pay the medical bills for people who can prove that their pets died from eating tainted food.

Backer said it does not take very long for the pets to get sick after eating the tainted food.