"The melamine finding came a week after scientists at the New York State Food Laboratory identified a cancer drug and rat poison called aminopterin as the likely culprit in the pet food. But the FDA said it could not confirm that finding, nor have researchers at the University of Medicine and Dentistry of New Jersey when they looked at tissue samples taken from dead cats. And experts at the University of Guelph detected aminopterin in some samples of the recalled pet food, but only in the parts per billion or trillion range.

"Biologically, that means nothing. It wouldn't do anything," said Grant Maxie, a veterinary pathologist at the Canadian university. "This is a puzzle."

New York officials stuck to their aminopterin finding and pointed out that it was unlikely that melamine could have poisoned any of the animals thought to have died after eating the contaminated pet food. Melamine is used to make plastic kitchen ware and is used as a fertilizer in Asia.

An FDA official acknowledged that it wasn't immediately clear whether the melamine was the culprit. The agency's investigation continues, said Stephen F. Sundlof, director of the FDA's Center for Veterinary Medicine."