The investigation into the source of the cyclosporiasis outbreak is going back to the drawing board after the Food and Drug Administration said a sample of Taylor Farms shredded lettuce produced a false positive result.

Taylor Farms, a California-based produce farm and supplier, announced a voluntary 27-state recall of its shredded iceberg lettuce on Friday after officials linked the product to the outbreak. The FDA had come out with a positive cyclospora result for Taylor Farms’ iceberg lettuce on Saturday, but declared it a false positive the following day.

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