Investigators, meanwhile, are looking into whether the rat poison came into the United States on an ingredient used in the recalled food. ABC News has learned that Menu Foods bought wheat gluten, the only ingredient changed in its plants, from China. That possibility raises questions about the safety of pet and other food products in the United States.
The chemical can cause kidney failure in dogs and cats, said Donald Smith, dean of Cornell University's College of Veterinary Medicine, during the conference.
Scientists at the state's food laboratory in Albany, N.Y., made the discovery a week after Menu Foods issued its recall.
"This is one step in a long process that will lead us to know what has happened and how it has happened," Smith said.
Aminopterin is not registered for use as a pesticide in the United States, and it has been known as a potential source of birth defects in humans.
ABC News first reported that the rodenticide may have been present in the wheat that was imported from China and used by Menu Foods, according to a source close to the investigation.
Investigators also have not yet determined whether aminopterin is the only contaminant in all the recalled food.
"If it is not the only culprit, as I suspect, the problem isn't over," McGill said, adding that it is also uncertain as to whether the finding will be much help to veterinarians.
According to McGill, even if the aminopterin is the culprit, "most veterinarians have never heard of this product. There will need to be more information put out to suggest therapeutic regimens."