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U.S. food regulators say they weren’t able to identify a contamination source for a food poisoning outbreak tied to romaine lettuce this fall.

The Food and Drug Administration says it couldn’t determine how a water reservoir on a Santa Barbara County, Calif., farm became contaminated with E. coli or how lettuce from other farms may have been contaminated. The FDA says leafy greens’ short shelf-life makes it difficult to investigate outbreaks. It notes food safety has been an issue with leafy greens, and the industry should review operations to minimize risk.

The Centers for Disease Control and Prevention says there is always risk of foodborne illness when eating raw produce.