At least one school district has already warned parents that food served in its cafeterias was recalled because of possible E. coli contamination.

WSJ: Recalled frozen food may have ended up in schools

Rich Products two weeks ago announced a voluntary recall of certain Farm Rich and Market Day brand products because of the possible E. coli contamination.

WSJ: Recalled frozen food may have ended up in schools

On Tuesday, Hudson recalled 20, 000 pounds -- 80, 000 frozen hamburger patties -- after 16 Colorado consumers became sick from grilled burgers linked to E. coli contamination.

CNN: Recall of frozen hamburger patties doubles