CINCINNATI -- First it was the tomatoes. Now it's the beef.

Fourth of July picnic tables are getting a careful look as familiar hamburgers are feared to be among the tainted ingredients in separate food safety scares.

On Wednesday, The Kroger Co. expanded its voluntary recall of some ground beef products to its stores in more than 20 states, including both Virginia and West Virginia, 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 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.

Kroger said Wednesday that as a precaution it removed from stores all ground beef supplied by Nebraska Beef marked with sell by dates of May 21 or later.

"Ground beef in stores today comes from other suppliers not involved in the recall," Kroger spokeswoman Meghan Glynn said Wednesday.

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.

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