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."

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