I'll bet you find that the culprits don't take the necessary or appropriate steps to ensure that their workers have a legal right to work in the US, either. It's the same greedy, corner-cutting, anything-for-a-buck mentality.
Actually, the FBI seems like the wrong agency to investigate food contamination, unless it was suspected to be domestic bioterrorism. I would suspect a USDA or FDA agency to be the principal investigators.