The Colorado Department of Public Health is investigating an E. coli outbreak at three metro-Denver Jimmy John's restaurants.

According to the Huffington Post, at least eight people were sickened after eating sandwiches at Denver-area Jimmy John's. The health department said only one individual has been hospitalized and all have recovered.

The incident occurred at Jimmy John's between Oct. 5 and 9. Customers became sick between the Oct. 7 to Oct. 15, and the cases were reported on the Oct. 18. 

The Centers for Disease Control and the Food and Drug Administration are currently investigating the outbreak, but none of the restaurants have been closed, nor identified. 

"Our leading hypothesis for what's happened is that there was a contaminated produce item that was distributed to the stores," Alicia Cronquit, an epidemiologist with the Colorado Department of Public Health and Environment told KDVR. "We have not identified any food handling issues at the particular establishments that we think would contribute to illness."

Most E. coli (Escherichia coli) strains are harmless and are an important part of a healthy human intestinal tract. Symptoms of E. Coli can include bloody diarrhea, severe stomach cramps and vomiting. 

The FDA is currently conducting tests to see where the contaminated produce came from. The department of health says Jimmy John's has been cooperating throughout the investigation.