Chipotle Mexican Grill Inc. closed 43 restaurants in Seattle and Portland, Oregon, as health officials are investigating an E. coli outbreak linked to the Mexican food chain.

All 43 restaurants in the areas have been shut after six of them were found connected to the two-week outbreak, during which about 20 people became sick, the Denver-based company said in an e-mailed statement.

Three people in the Portland area and 19 in Washington state became ill after eating at the Chipotle restaurants since Oct. 14, according to a statement posted Saturday on the website of the Oregon Health Authority. One-third of those affected have been hospitalized and there have been no deaths, it said.

The illness is believed to be caused by the Shiga toxin E. coli, with symptoms including vomiting and bloody diarrhea. Chipotle is “working with health department officials to determine the cause of this issue,” spokesman Chris Arnold said.