The Marin County Department of Health and Human Services and Environmental Health Services has concluded its investigation of an outbreak of E. coli last month that sickened six people, four of whom were children.

No single source of the infection has been identified and there have been no new cases since mid-September, according to a press release. There are up to 10 documented cases of E. coli a year in Marin, usually in isolated cases or small clusters.

Two children and two adults were hospitalized, but have since been discharged.