United States food safety regulators say they are still mystified by an outbreak of E coli that has killed one person, sickened 157 and forced all fresh spinach to be pulled from store shelves.

The investigation centers on nine farms in three California counties, and the outbreak may signal a need for tighter regulation – especially in California’s crop-rich Salinas Valley, an FDA official said. California health officials, the FDA and an investigator from the US Centers for Disease Control and Prevention are inspecting nine farms in California’s Monterey, San Benito and Santa Clara counties.

Dr David Acheson, chief medical officer for the agency’s Food Safety and Applied Nutrition branch, said presumably fresh spinach from elsewhere was safe but the agency needed to come up with clear language to guide consumers so they would be confident buying fresh spinach again.

The contamination could have come from water, manure, a breakdown in the packaging plant, or improperly refrigerated spinach, Acheson said. The farms are the likely source of the problem. "We are looking at drainage, we are looking at irrigation, we are looking at the topography of land," Acheson said. If good agricultural practices are not being followed, then perhaps there is a need to tighten up the voluntary aspects.

This is the 10th outbreak of E coli to be traced to the Salinas Valley area.