FDA import alert - January 2008

In April 2007, U.S. FDA took samples and tested vegetable protein imports after thousands of pets became sick or died from eating pet food contaminated with melamine. 

In January 2008, U.S. FDA issues "Detention without physical examination" notice for vegetable proteins entering the U.S. for animal or human consumption.

The notice states, "FDA does not know how widespread the problem in China might be. For example, FDA does not know which regions of the country may or may not be impacted by the problem, which firms are the major manufacturers and exporters of vegetable proteins to the United States, where these vegetable proteins are grown in China, and what controls are currently in place to prevent against contamination." 

I guess it's just easier to set a Parts Per Million threshold for poison in food than to trace it and try to eliminate the poison from the food supply.