SEATTLE — An E. coli outbreak linked to Chipotle restaurants in Washington state and Oregon has sickened nearly two dozen people in the third outbreak of foodborne illness at the popular chain this year.

Cases of the bacterial illness were traced to six of the fast-casual Mexican food restaurants, but the company voluntarily closed down 43 of its locations in the two states as a precaution — all those in Washington state and in the Portland, Oregon, area.

Three people in the Portland area and 19 people in western Washington have gotten sick with E. coli as of Friday. Seventeen of them had eaten at a Chipotle restaurant during the past few weeks. Eight people have been hospitalized but no deaths have been reported.

Health officials are investigating the cause of the outbreak. The source was most likely a fresh food product because it probably could not be traced to one sick individual or one instance of cross-contamination of food since the cases are connected with various restaurants, said Marisa D’Angeli, medical epidemiologist with the Washington State Department of Health.

The company is not planning to close any other restaurants in other states because there is no evidence of a link to other locations, company spokesman Chris Arnold said. Reopening the shuttered locations in Oregon and Washington will depend on the investigation, he said.