How did the FDA discover that some vaccine manufacturers are not universally following letters, Points to Consider (PTC) and guidance documents on sourcing of bovine materials?

During review of new license applications manufacturers are asked to provide detailed descriptions of the manufacturing process and documentation of source country for all materials of animal origin. In 2000, during review of a license application it was determined that some of the material used during manufacture had been obtained from countries which are on the USDA list of countries which either have or may have BSE. This finding prompted an inquiry of all licensed vaccines.