From Patricia Doyle, PhD

It appears that Cyanuric acid, a chemical used to extend the life of chlorine in swimming pools and melamine were both found in the contaminated pet food. When these chemicals are together they act as a binary weapon, they become deadly.

The following was from Animal Net, a food safety net from Canada.

Just think how easy it would be for China to deliberately poison our food. Simple really, and it would appear as an innocent food additive problem as we hear with the pet food scandal.