Source of fall romaine outbreak a mystery: U.S. regulators

U.S. food regulators say they weren't able to identify a contamination source for a food poisoning outbreak that prompted them to warn people to avoid romaine lettuce this fall.

The Food and Drug Administration says it wasn't able to determine how a water reservoir on a Santa Barbara County, California, farm became contaminated with E. coli, or how lettuce from other farms may have been contaminated.

Romaine calm: We now have a recall, but…

The FDA says leafy greens' short shelf-life makes it difficult to investigate such outbreaks. It notes food safety has been a longstanding issue with leafy greens, and that the industry should review operations to minimize risk.

READ: Romaine lettuce ‘particularly susceptible’ to E. coli outbreaks

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