ALBANY, N.Y. — Rat poison was found in the pet food suspected of causing kidney failure that killed at least 16 cats and dogs, but scientists still don't know how it got there, state officials said Friday.

The toxin was identified as aminopterin, which is used to kill rats in some countries, state Agriculture Commissioner Patrick Hooker said. Aminopterin is not registered for killing rodents in the U.S., according to the Environmental Protection Agency, though it is used as a cancer drug.

State officials wouldn't speculate on how the toxin got into Menu Foods' now-recalled pet food but said no criminal investigations had been launched.

Scientists at the New York State Animal Health Diagnostic Center at Cornell and at the New York State Food Laboratory tested three cat food samples provided by Menu Foods and found Aminopterin in two of them. Hooker said they would test individual components of the pet food, as well. The early test results were released to give veterinarians a better idea of how to treat sick animals.

"Any amount of this product is too much in food," Hooker said.

Aminopterin, also used as a cancer drug, is highly toxic in high doses. It inhibits the growth of malignant cells and suppresses the immune system.

In dogs and cats, it can cause kidney failure, according to Donald Smith, dean of Cornell University's College of Veterinary Medicine.

Cases of kidney failure connected to the food have been reported in Wilson, Durham, Orange and Wake counties. A statewide survey has turned up 33 cases scattered across North Carolina.

Ryan Eiland and his wife lost one cocker spaniel earlier this week from liver and kidney failure that he blames on the tainted food, and another of their dogs is suffering similar symptoms. The Morrisville resident said he fed all the of his animals food that was on on the recall list that he bought from a local Petco store.

Eiland says they've already racked up hundreds of dollars in veterinary bills, and he said he's worried other animals might go untreated if their owners can't afford it.

"I think it's absolutely horrendous that this food has made it into the food chain and that it's gone undetected for this amount of time," he said. "For them to finally acknowledge the source of the problem, I think immediatly that these companies should be setting up vet care."

The Food and Drug Administration has said the investigation into the pet deaths was focusing on wheat gluten in the pet food. Wheat gluten itself would not cause kidney failure, but the common ingredient could have been contaminated, the FDA said.

Bob Rosenberg, senior vice president of government affairs for the National Pest Management Association, said he had never heard of the substance before Friday.

"It would make no sense to spray a crop itself with rodenticide," Rosenberg said, though he said grain shippers typically put bait stations around the perimeter of their storage facilities.

The pet deaths led to a recall of 60 million cans and pouches of pet food produced by Menu Foods and sold throughout North America under 95 brand names -- each brand name includes various products. There have been several reports of kidney failure in pets that ate the recalled brands, and the company has confirmed the deaths of 15 cats and one dog.

Menu Foods last week recalled "cuts and gravy" style dog and cat food. The recall sparked concern among pet owners across North America. It includes food sold under store brands carried by Wal-Mart, Kroger, Safeway and other large retailers, as well as private labels such as Iams, Nutro and Eukanuba.

"You might want to avoid these gravy, chunky-style types of pet food for your dog or cat. Dry food appears to be safe," said Dr. Richard Ford, a clinical sciences professor at North Carolina State University's College of Veterinary Medicine.

Some local pet owners said they are considering a switch organic pet food.

"I'd rather be safe than sorry and switch," Anne Campbell said.

There is no antedote for the type of rat poison discovered in the pet products, Ford said. The best veterinarians can do is stabilize a pet to minimize the damage to internal organs and make the animal as comfortable as possible.

A complete list of the recalled products along with product codes, descriptions and production dates was posted online by Menu Foods and is available at http://tinyurl.com/2pn6mm. The company also designated two phone numbers that pet owners could call for information: (866) 463-6738 and (866) 895-2708.

A spokesman for New York Attorney General Andrew Cuomo said he was not aware of any criminal investigation involving the tainted food. FBI spokesman Paul Holstein in Albany said Friday he was not aware of any FBI involvement in the case.

"I don't know where we'll go from here," he said.

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Associated Press writer Andrew Bridges in Washington contributed to this report.

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