Update: The pet food scare has widened. As of Friday night, at least one brand of dry cat food has been recalled: Prescription Diet m/d Feline, which is made by Hills Pet Nutrition, Inc., of Topeka, Kan. This kind of pet food is typically sold online and by veterinarians, in four- and 10-pound bags. Earlier this year, the company used wheat gluten from the same supplier that sold the protein to Menu Foods, Inc., which recalled more than 60-million cans and pouches of "cuts and gravy" style wet food two weeks ago, after receiving complaints and conducting "tasting trials" on animals.

As the scare widens, so does the probe: The Food and Drug Administration (FDA) says it is considering regulatory changes to tighten controls and is investigating whether any of the contaminated wheat gluten—which is implicated in more than 100 animal deaths—made it into food consumed by humans. The FDA and the Cornell University College of Veterinary Medicine announced today that new investigations uncovered another contaminant, melamine, in samples of food linked to the deaths of over 100 animals, a number that officials fear will climb.

In light of the new findings, FDA officials say concern has shifted to melamine, a chemical used in industrial and commercial plastics, as a possible culprit. Multiple labs found melamine in food samples as well as in the tissue of sick animals. This latest discovery comes a week after a lab in Albany, N.Y., found high concentrations of aminopterin (a rat poison) in samples from over 60 million cans and pouches of moist meals recalled earlier this month by Canadian company Menu Foods. That finding has yet to be consistently replicated in other labs, including those of Cornell and the FDA. Aminopterin is a toxin known to cause acute kidney failure, the very same affliction reported over the past two weeks in nearly 500 cats and dogs across the nation that ate the recalled food.

"Melamine is an ingredient that should not be in pet food at any level," Stephen Sundlof, director of the FDA's Center for Veterinary Medicine, said at a press conference, adding that the agency is "not yet fully certain that melamine is the cause of illness and death in pets."

Investigators found the chemical in food samples and in the urine of some affected cats as well as in the kidney of a cat that died after eating tainted food. Investigators also found melamine in the same ingredient that they had previously discovered the rat poison: wheat gluten, a protein used to thicken the wet food's gravy. Menu Foods has said that it had switched to a wheat gluten vendor in China just before the problems began cropping up. The company says it has since returned to its former gluten supplier.

Melamine is a urea-based product used in industrial and commercial plastics, like eating utensils and laminates, such as whiteboard wall paneling, flooring and Formica countertops. According to Sundlof, it is also used as a fertilizer in Asia. He notes, however, that, "at this time, we do not know how the melamine got into the wheat gluten."

Officials earlier speculated that aminopterin got into the pet food because Chinese farmers sprayed it on wheat fields to protect them from rodent trespassers.

Michael Rogers, director of the FDA's Division of Field Investigations says the wheat gluten vendor has been contacted and that "the agency is initiating a 100 percent review and sampling of all imported wheat gluten from China" entering the U.S.

A food safety lab at the New York State Department of Agriculture & Markets (NYSADM) in Albany last week announced that it had found aminopterin in food samples provided by Menu Foods. Aminopterin was used in the U.S. in the 1940s as a cancer drug, but is currently not approved for use in North America for anything other than lab research. China and some other countries, however, use it as rodent poison; rats exposed to it die from kidney failure— the same cause of death of the dogs and cats that ate the recalled grub. The concentration of the toxin found by the NYSADM, was 40 parts per million (ppm); the lethal dose in rats is three ppm.

Officials refused to disclose the amount of melamine found in the food. Richard Goldstein, a Cornell University veterinarian and kidney specialist, says there is "nothing in the literature to suggest that melamine would cause the [kidney] lesions that we're seeing" in the sick pets. He adds that melamine is not known to be "a very toxic compound," but notes there may be harmful by-products of the substance. Still, he believes, "It is easier to see the connection between aminopterin [rather] than with melamine," and the pet deaths, based on the observed symptoms.

Donald Smith, dean of Cornell's vet school, says the ill pets typically have spherical, brown crystals of unknown composition in their urine and, in some cases, in their kidneys as well. He notes that the linings of their kidney tubules (where urine forms and collects before being sent to the bladder) were also damaged.

The presence of both aminopterin and melamine raises disturbing questions about the safety of pet food and the potential of such an incident happening again.

Asked whether the agency could have detected the chemicals before they got into the food, Sundlof says it was "unlikely," adding that inspectors did not find any safety violations when they examined Menu Foods's facilities in Kansas and New Jersey just days before the recall was announced to the public. Because the contamination is "in a normal component of food," Sundlof says, "it would not have likely been picked up during inspection."