Almost three weeks into their investigation of an E. coli outbreak in and around St. Louis, public health officials are no closer to finding a source for the pathogen.

The Missouri State Public Health Laboratory has confirmed that at least 31 people have become ill from E. coli bacteria that health officials have said from the beginning is definitely foodborne. A total of 49 people had been tested as of Nov. 9. The outbreak is estimated to have begun Oct. 24.

Initially officials thought fresh produce on salad bars at Schnucks Markets might have been implicated, but so far 53 food samples from the salad bars and two samples from patients’ homes have all returned negative test results.

The Centers for Disease Control and Prevention sent a team of four investigators to St. Louis Oct. 28 to help with the investigation. Investigators from the Food and Drug Administration joined the search for a source during the first week of November to check out-of-state produce suppliers.

Leading the investigation is the Missouri Department of Health, which has been issuing daily news releases with lab result tallies. New suspected cases have been declining and state officials said the worst of the outbreak is likely over because E. coli has an incubation period of only about 10 days.