WASHINGTON, DC (WRIC) — A salmonella outbreak that infected 31 people across the Great Lakes region last Summer has been traced to a hydroponic farm that produced salad greens, according to the FDA.

The FDA originally recalled packaged salads across five states after people began falling ill in July.

Now, an investigation completed by the FDA and CDC found that the contamination likely originated in a hydroponic growing facility owned by BrightFarms, LLC.

Hydroponic farming is a technique in which plants are grown with their roots suspended in nutrient-rich water, instead of traditional planting. Hydroponic farming is efficient – but as the FDA investigations hows, can also present a risk of salmonella contamination.

Testing performed by the FDA revealed concentrations of Salmonella in a stormwater basin near the farm, but did not determine if it was the source of the contamination. In a press release following the investigation, the FDA emphasized the importance of general sanitation and microbial testing – not only in facilities themselves, but in nearby water sources.