Company officials said the meat might be contaminated with E. coli.

The warning comes as federal investigators try to pinpoint the source of a separate salmonella outbreak linked to tomatoes that 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 but remain mum on exactly what vegetables are getting tracked.

Kroger's recall stems from meat obtained from one of Kroger's suppliers, Nebraska Beef Ltd., that has been linked to illnesses reported in Michigan and Ohio between May 31 and June 8 caused by E. coli bacteria.

Nebraska Beef has recalled from wholesalers and other processing companies nearly 532,000 pounds of ground beef produced on five dates between May 16 and June 24.

As a precaution, Kroger removed from stores all ground beef supplied by Nebraska Beef marked with sell by dates of May 21 or later.

The Cincinnati-based company initiated a recall June 25 for Kroger stores in Michigan and in central and northern Ohio.

The expanded recall includes ground beef sold at Fred Meyer, QFC, Ralphs, Smith's, Baker's, King Soopers, City Markets, Hilander, Owen's, Pay Less and Scott's with overlapping sell-by dates from mid-May through mid-July.