Also today, the company that produced the food expanded its recall to include all 95 brands of the "cuts and gravy" style food, regardless of when they were produced.

The substance in the food was identified as aminopterin, a cancer drug that once was used to induce abortions in the United States and is still used to kill rats in some other countries, state Agriculture Commissioner Patrick Hooker said.

The federal government prohibits using aminopterin for killing rodents in the U.S. State officials would not speculate on how the poison got into the pet food, but said no criminal investigations had been launched.

The pet deaths led to a recall of 60 million cans and pouches of dog and cat food produced by Menu Foods and sold throughout North America under 95 brand names. Some pets that ate the recalled brands suffered kidney failure, and the company has confirmed the deaths of 15 cats and one dog.

The company expanded the recall — which initially covered only cans and pouches of food packaged from Dec. 3 through March 6 — after the FDA alerted it that some products remained on store shelves.

There is no risk to pet owners from handling the food, officials said.

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

Bob Rosenberg, senior vice president of government affairs for the National Pest Management Association, said it would be unusual for the wheat to be tainted.

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

Scientists at the New York State Animal Health Diagnostic Center at Cornell University and at the New York State Food Laboratory tested three cat food samples provided by the manufacturer and found aminopterin in two of them. The two labs are part of a network created after the Sept. 11 terrorist attacks to keep the nation's animals and food supply safe.

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

Aminopterin is highly toxic in high doses. It inhibits the growth of malignant cells and suppresses the immune system. In dogs and cats, the amount of aminopterin found — 40 parts per million — can cause kidney failure, according to Bruce Akey, director of Cornell's diagnostic center.

"It's there in substantial amounts," Akey said.

Donald Smith, dean of Cornell's veterinary school, said he expected the number of pet deaths to increase. "Based on what we've heard the last couple days, 16 is a low number," Smith said.

Aminopterin is no longer marketed as a cancer drug, but is still used in research, said Andre Rosowsky, a chemist with the Dana Farber Cancer Institute in Boston.

Rosowsky speculated that the substance would not show up in pet food "unless somebody put it there."

Paul Henderson, chief executive of Ontario, Canada-based Menu Foods, said the company does not believe the food was tampered with because the recalled food came from two different plants, one in Kansas, one in New Jersey. Menu continues to produce food at the two plants.

The company, already facing lawsuits, said today it is testing all the ingredients that go into the food.

"We have a lot of work to do, and we are eager to get back to it," Henderson said. "This is a highly unusual substance."

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.

Associated Press writers Andrew Bridges in Washington and Michael Virtanen in Albany contributed to this report.

On the Web:

FDA pet food recall information: http://www.fda.gov/oc/opacom/hottopics/petfood.html