I've read that wheat gluten is the suspect ingredient. It's used to create the "gravy" in soft pet food. I'm surprised there has been so little speculation about tampering. The culture we're at war with is not fond of "pets" per se. We, on the other hand, are quite fond of them. Safeguards on food products for human consumption strike me as too relaxed; I can imagine that they're next to nonexistent for pet food.
Pets are important to us Americans and I'm sure that terrorists are aware of it.
Interestingly enough, pet food controls are more strict than human food. People get more upset when Fluffy gets sick than Sue.