The Kroger Company is expanding its voluntary recall of some ground beef products to its stores in more than 20 states, saying the meat may be contaminated with E. coli.

The nation's biggest traditional grocer also urged customers to check the ground beef in their refrigerators and freezers to determine whether it is covered by the recall.

The warning comes as federal investigators try to pinpoint the source of a separate salmonella outbreak linked to tomatoes.

That outbreak has sickened nearly 900 people, raising more questions about the nation's food safety system.

While insisting that tomatoes remain the leading suspect, investigators are looking at other produce.

Kroger's recall stems from meat obtained from one of Kroger's suppliers, Nebraska Beef.

Kroger says the sell-by dates on its recalled beef are May 21st or later.