Rat poison was identified Friday as the substance suspected of contaminating pet food that has killed or sickened dogs and cats across the nation, although it is still unclear how the deadly chemical got into the food.

Federal officials, meanwhile, reported an expanded recall of dog and cat food produced by Menu Foods of Ontario, Canada. The company last weekend voluntarily recalled 60 million cans and pouches of potentially contaminated products — including popular brands such as Iams and Eukanuba and private-label brands sold by large retailers.

The expanded recall and the findings released by New York state health officials raised concerns that the death toll from the contaminated food could rise significantly as more cases come to light.

The Food and Drug Administration estimated that the contaminated food has caused 14 pet deaths in the United States. However, the agency said it had received 4,400 complaints and inquiries from pet owners and veterinarians about the matter, and others say the death toll may be higher.

As of midday Friday, the Los Angeles County Department of Public Health reported 10 confirmed pet deaths — five cats and five dogs — from kidney failure in recent days and 15 additional cases of very sick animals with similar symptoms.

Dr. Jonathan Fielding, the county’s public health director, said the department was investigating 86 suspicious cases in which pet food might have played a role in an animal’s recent illness.

Health officials in New York said laboratory tests of pet food made by Menu Foods found aminopterin, a substance that is used to kill rodents in other countries but is banned in the United States. The tests found levels of at least 40 parts per million.

The pet food samples were tested by the New York State Animal Health Diagnostic Center at Cornell University and the New York State Food Laboratory, which identified aminopterin as the toxic substance. The labs are part of a nationwide network set up after the 2001 terrorist attacks to test for contaminants in the nation’s food supply.

Executives at Menu Foods said Friday that they didn’t know how the rat poison got into their products. They said they had stopped using an ingredient that might be responsible for the contamination. The company declined to say what it was.

Because aminopterin is widely used as a rodenticide in China, where Menu Foods gets wheat gluten that is used to make pet "gravy" for some of its pet-food brands, there is some suspicion that the country could be the origin of the problem.

Although the FDA said it hadn’t located the exact source of the contamination, it did say it had traced the delivery route of wheat gluten supplied to the Menu Foods plants in Kansas and New Jersey where the contaminated food was produced. Menu continues to produce pet food at the two plants.

FDA officials also said there had been no reports of human sickness in connection with the tainted products.

The agency said Menu Foods had voluntarily expanded its recall to include all 95 brands of its "cuts and gravy"-style food, regardless of when they were produced. Previously, the recall included products made from Dec. 3 of last year to March 6.

Dry pet food is not included in the recall.