People are becoming ill three weeks after the U.S. Food and Drug Administration warned consumers to avoid certain tomatoes linked to the outbreak, Robert Tauxe with the Centers for Disease Control and Prevention in Atlanta told USA Today.
As of Monday, there were 851 reported cases, some indicating people fell ill as recently as June 20, said Tauxe, deputy director of the CDC's division of foodborne diseases.
"We're broadening the investigation to be sure it encompasses food items that are commonly consumed with tomatoes," Tauxe said.
If another source is found to be the culprit after tomatoes were recalled and the produce industry sustained losses in the hundreds of millions of dollars, food safety experts said trust in the government's ability to track foodborne illnesses will erode.
"We can't let this investigation, however it might turn out, end with just the answer of 'What caused it?'" said Michael Osterholm of the Center for Infectious Disease Research and Policy. "We need to take a very in-depth look at foodborne disease investigation as we do it today."