Blue Bell Creameries’ ice cream manufacturing facility in Broken Arrow, Oklahoma, has not made a carton of ice cream for over a month. Production ground to a halt on April 3 after the U.S. Food and Drug Administration found the bacteria known as listeria in a single serving cup of chocolate ice cream that originated at the facility. New evidence revealed Thursday from a federal investigation suggests listeria was first detected at the plant in 2013 and investigators stated that the company has failed to adequately address the problem.

Ten people have been infected with listeria from Blue Bell products in recent months, and three have died. Blue Bell officials stated that the intent of the plant closure is to determine the cause of contamination. The FDA adds that the company has pledged to undergo “an intensive cleaning and training program at all of its production facilities” after the agency also found sanitation problems at production facilities in Texas and Alabama.

The outbreak seems to have stemmed from a chronic cleaning problem at the Broken Arrow facility. A team of FDA inspectors who visited the plant found listeria on 17 separate occasions between March of 2013 and February 2015. They wrote in a report: “The procedure used for cleaning and sanitizing of equipment and utensils has not been shown to provide adequate cleaning and sanitizing treatment.”

One of the most common places for listeria to dwell in a food processing facility is in floor drains. Based on the FDA report, these drains were a likely culprit in this outbreak. In March 2013, FDA investigators found listeria on the floor in front of a freezer at the Broken Arrow facility. They found it there again in October. In 2014, the bacteria was discovered lurking in a drain and a water hose. By January and February of 2015, it had returned to the floor in front of one or more freezers.  

Floor drains are a prefect breeding ground for listeria because they provide the soils necessary for the bacteria to grow, as explains. They are often difficult or unpleasant to clean and so can go neglected by cleaning staff for long periods of time. Once listeria takes hold in a drain, the bacteria can make its way onto the floor and be picked up and transferred to other surfaces by passing equipment, people or pests. High-pressure sprays can also blast these organisms into the air and allow them to fall onto surfaces that are in turn used to process food.

The FDA offers non-binding recommendations to the frozen food industry on how to keep the threat of listeria at bay, and emphasizes the importance of cleaning drains and floors. Typically, staff in a facility where listeria has been detected will treat drains with a cleaner that breaks up the soil that clings to the sides, thereby removing the bacteria’s primary food source.

Dr. Robert Tauxe of the Centers for Disease Control and Prevention says that contaminated equipment is another common culprit in listeria outbreaks, the Dallas Morning News reports. The FDA reports several pieces of equipment at the Broken Arrow plant were contaminated including a pallet jack, half-gallon fillers, a pint filler and a cleaning tub. Staff may have to disassemble and deep clean all of the affected equipment to fully remove the threat.

Paul Kruse, CEO of Blue Bell, has said that he expects to return to the business of making ice cream at the Broken Arrow plant only after “several months” of inspection and cleaning, BuzzFeed reports.