CINCINNATI (AP) — 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, 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.

In some stores, the recall includes products in Styrofoam tray packages wrapped in clear cellophane or purchased from an in-store service counter. It does not include ground beef sold in 1-, 3-, or 5-pound sealed tubes or frozen ground beef patties sold in the frozen food section of its stores.

Kroger is notifying customers about the expanded recall by placing signs in stores in meat departments. It also is using its register receipt notification system.

Kroger can track purchases by customers who use the company's loyalty card, which entitles customers to certain discounts. Sometimes those customers receive information about products the next time the card is used and a receipt is issued, Glynn said.

In other cases, Kroger is able to call customers who used the loyalty card to purchase a tainted product, and it is doing that with the ground beef recall, Glynn said.

Symptoms of E. coli infection can include severe stomach cramps, diarrhea, vomiting and fever. It can potentially be deadly, but most people recover within five to seven days.

Health officials urge people to thoroughly cook hamburger and, if possible, use a digital thermometer to make sure meat has been heated to at least 160 degrees.

They also recommend that people wash their hands with warm water and soap for at least 20 seconds before and after handling food.

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Consumers who have questions about the recall may contact Kroger toll-free at 800-632-6900 or online at http://www.kroger.com/recalls.

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On the Net:

USDA's Food Safety and Inspection Service: www.fsis.usda.gov

U.S. Centers for Disease Control E. coli site: http://cdc.gov/ecoli