Aminopterin sounds like that's it. I was thinking resin earlier when melamine was mentioned, but melamine itself is just a condensation product of urea and ammonia used as fertilizer also. It hydrolyzes with water and goes back to urea and ammonia. The LD50 is 3.2 grams/kg rat, mouse, so it's not very toxic. Depending on the concentrations and ratios they find, they can tell how the contamination likely occurred. I doubt there was much melamine, because it would taste disgustingly bitter and ammonical. It would only take small amounts of
aminopterin.