BURGERS AND FRIES ARE SAFER THAN SALAD: What Sparked An E. Coli Outbreak In Lettuce? Scientists Trace A Surprising Source. “Investigators quickly identified romaine lettuce as the source of the outbreak, but have had trouble pinpointing the cause for months. Now, the Food and Drug Administration has a theory for how E. coli ended up on that lettuce. According to the FDA, it probably came from a large cattle feedlot at one end of a valley near Yuma, Ariz., which is one of the country’s biggest lettuce-growing areas.”