Dole Food Company is officially under criminal investigation by the U.S. Department of Justice following a Listeria outbreak that claimed the lives of four people and sickened at least 29 others in the U.S. and Canada.

The outbreak was traced to Dole’s packaged salads after swab samples from both the victims and product samples were evaluated by officials.

The presence of Listeria in Dole’s Springfield, OH plant dates back to July 2014. Contamination was also discovered at the plant in 2015, suggesting that the plant continued to operate without addressing the food safety violation. The plant was forced to shut down in January after samples collected by the U.S. Food and Drug Administration and the Canadian Food Inspection Agency tested positive for Listeria. The plant reopened in April after updating sanitation methods and procedures. For now, production is limited and will continue to expand to normal capacity over the next few weeks.

Some reports allude to Dole’s deliberate masking of the plant’s Listeria presence. According to federal investigators, Dole failed to test surfaces that came into direct contact with food. Also, Dole apparently tested areas of the plant before food production and sanitizing took place, making it more difficult to determine whether or not the required sanitation procedures really worked or not.