It's my understanding just from reading news reports that the aminopterin AND melamine were found, in the samples they tested, by the New York State Food Laboratory and the New York State Animal Health Diagnostic Center at Cornell University. The FDA are the ones, I think, who found the melamine, but didn't find the aminopterin in the samples they tested.

Someone correct me if that's wrong, please.

edited to add more.

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The Cornell Daily Sun
C.U. Helps Identify Poison in Pet Food
By Willimina Bromer
Sun Contributor
Mar 28 2007

says this "The Animal Health Diagnostic Center at Cornell worked in close collaboration with the New York State Food Laboratory to pinpoint the rat poison Aminopterin as the contaminant.

According to Dan Rice, director of the Food Laboratory, the AHDC sent the samples to the food laboratory for specialized testing, which is not conducted at Cornell’s lab."

Edited again: More...

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Scientific American
March 30, 2007
Pick Your Poison: Pet Recall Investigation Turns Up New Contaminant
Chemical used to make plastics is found in recalled food and sick animals
By Nikhil Swaminathan