Michigan and Illinois officials have issued warnings concerning bagged chopped shredded iceberg lettuce distributed by Aunt Mid's Produce Co. in Detroit. The lettuce might be contaminated with E. coli O157:H7.

CFIA officials said they're working with the Public Health Agency of Canada, the Ontario Ministry of Health and Long-Term Care, local public health authorities and U.S. officials on the investigation.

The iceberg lettuce was imported into Ontario for use in restaurants and institutions.