When first announced in May, the outbreak was thought be linked to cucumbers grown in Spain, though German health officials later deemed that to be incorrect. Lettuce and tomatoes from northern Germany were also implicated, but those too were not deemed the source of the outbreak. As illnesses and deaths grew, on June 5 German officials announced that bean sprouts from an organic farm in northern Germany were likely the offending food. On June 6, officials said initial tests provided no evidence that the bean sprouts were behind the outbreak.
On June 10, The Robert Koch Institute, Germany's national disease control center, lifted warnings against eating tomatoes, cucumbers, and lettuces and ruled that vegetable sprouts from an organic farm in Germany were the cause of the outbreak. Though June 6 testing had found no E. coli contamination at a sprout farm in Lower Saxony, an epidemiological trace-back investigation revealed sprouts to be a common food consumed by many of the victims.