Dole Food Company officials knew about the listeria problems at their salad plant in Springfield, Ohio more than a year before the factory closed in January, a report from the U.S. Food and Drug Administration (FDA) found.

The FDA's inspection report found that the surface tests at the Ohio plant tested positive for listeria nine separate times in 2014 and 2015, prior to the FDA's testing in January.

Craig Hedberg, an environmental health sciences professor at the University of Minnesota's School of Public Health, said that a persistent contamination could indicate a problem with sanitation.

Early this year, a listeria outbreak affected multiple states in the United States. It was traced to the Dole plant in Ohio where the packaged salads are made.

On Jan. 22, Dole issued a product recall on all salads packaged at the Springfield plant with the following brand names: Dole, The Little Salad Bar, Simple Truth, President's Choice, Fresh Selections and Marketside. The operations at the plant were also temporarily suspended.

"Those FDA reports deal with issues at our plant that we have corrected. We have been working in collaboration with the FDA and other authorities to implement ongoing improved testing, sanitation and procedure enhancements, which have resulted in the recent reopening of our Springfield salad plant," said Dole in a company statement.

The plant resumed operations on April 21.

The U.S. Centers for Disease Control and Prevention (CDC) said in January that it had received reports about the listeria cases as early as July 2015. The investigation started in September but it was only in January this year that the source was discovered.

To date, four people have died due to the listeria outbreak, which sickened 33 individuals in the United States and some as far off as Canada. Everyone who was sickened was hospitalized.

Dole said in a statement that the U.S. Department of Justice has contacted them in connection with the listeria outbreak. The company said it will be cooperating with the federal agency to address any more concerns.

On a similar note, the DOJ also started investigating Blue Bell Creameries in January for the alleged mishandling of an ice cream contamination issue. The event is associated with 10 listeriosis cases wherein three people died because of complications after eating Blue Bell frozen treats.

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