Illinois state health officials have traced a widespread Salmonella outbreak at Subway restaurants to one Central Illinois food distributor. But, in a final report on the incident issued Thursday, the Illinois Department of Public Health said it could not identify the exact source of the problem, which led to 109 confirmed cases and another 90-plus probable or suspect cases of the illness between late April and June.

In the 69-page report, investigators said the “most likely source” of the lettuce, tomatoes and olives linked to the illnesses was Lincoln-based Sysco Central Illinois Inc., which delivered produce to the affected restaurants.

More than 480 workers at the stores had to be tested. A dozen of them were found to be positive for the Hvittingfoss strain of Salmonella. Sangamon County had the most cases with 14, followed by Christian and Ogle counties with nine each.

In the end, 49 restaurants in 28 Central and northern Illinois counties were connected to the outbreak. More than two-dozen people were hospitalized.