FILE - This undated photo shows romaine lettuce in Houston. On Friday, June 1, 2018, the U.S. Centers for Disease Control and Prevention said it wasn’t able to identify a contamination source for an E. coli outbreak that prompted it to warn people to avoid romaine lettuce in the fall of 2018. (Steve Campbell/Houston Chronicle via AP)
FILE – This undated photo shows romaine lettuce in Houston. On Friday, June 1, 2018, the U.S. Centers for Disease Control and Prevention said it wasn’t able to identify a contamination source for an E. coli outbreak that prompted it to warn people to avoid romaine lettuce in the fall of 2018. (Steve Campbell/Houston Chronicle via AP)
FILE – This undated photo shows romaine lettuce in Houston. On Friday, June 1, 2018, the U.S. Centers for Disease Control and Prevention said it wasn’t able to identify a contamination source for an E. coli outbreak that prompted it to warn people to avoid romaine lettuce in the fall of 2018. (Steve Campbell/Houston Chronicle via AP)

NEW YORK | 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 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.

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