A stomach bug known as cyclospora has sickened more than 300 people in 15 states.
Cyclospora is a rare parasite that causes a lengthy gastrointestinal illness.
State officials in Iowa and Nebraska said Tuesday that they believe a pre-packaged salad mix is linked to cases there.
But the Center for Disease Control said it's not convinced all the cases are linked to a single source.
"CDC is still actively pursuing all leads and hasn't implicated any single food item as the cause of the outbreak in all states," Sharon Hoskins, CDC spokeswoman, said. "We're still not sure if the cases in all of the states are linked to the same outbreak."
The infection causes diarrhea and other flu-like symptoms. Twenty-one people have been hospitalized.
The CDC said 372 cases of the cyclospora infection have been reported Iowa, Texas, Nebraska, Florida, Wisconsin, Illinois, New York, Georgia, Missouri, Arkansas, Connecticut, Kansas, Minnesota, New Jersey and Ohio.