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Food retailers and manufacturers in the U.S. are rushing to tell consumers that their products are safe amid nationwide recalls of more than a half-billion eggs that may be infected with salmonella bacteria.

The U.S. Food and Drug Administration on Thursday said it had not yet found clear evidence of contamination, although heavy rainfall near the Galt, Iowa company that produced most of the eggs may have raised the risk of salmonella infestation from rodents.

The company, Wright County Egg, is owned by Jack DeCoster, whose companies in the past have been fined for allegedly hiring illegal immigrants and for other alleged workplace violations. The company recalled 380 million eggs earlier this week.

Another Iowa company, Hillandale Farms, said Friday it was recalling more than 170 million eggs after laboratory tests confirmed salmonella.

The recalled eggs are linked to at least 1,300 illnesses in at least 10 states, and maybe more, according to the Centers for Disease Control and Prevention. Consumers should check the end of their egg cartons holding six, 12 and 18 eggs for batch numbers 1026, 1413 and 1946, with date codes ranging from 136 to 225.

The CDC is working with state health departments to investigate the illnesses. No deaths have been reported, said Dr. Christopher Braden, a CDC epidemiologist involved in the investigation.

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Initially, 228 million eggs were recalled but that number was increased to the equivalent of nearly 32 million dozen-egg cartons.

"We are conducting an extensive investigation," said Sherri McGarry of the FDA. "Once we were made aware of this nationwide outbreak of salmonella enteritidis, we investigated specific clusters of illness and were able to trace it back to a common source; the Wright County Egg operations in Iowa."

Minnesota, a state with some of the best food-borne illness investigators in the country, has tied at least seven salmonella illnesses to the eggs.

Other states have seen a jump in reports of the type of salmonella. For example, California has reported 266 illnesses since June and believes many are related to the eggs. Colorado saw 28 cases in June and July, about four times the usual number. Spikes or clusters of suspicious cases have also been reported in Arizona, Nevada, Illinois, Texas and Wisconsin.

The CDC said health officials have reported at least 1,953 cases of salmonella from May through July – a time period that usually sees only approximately 700 reports. It’s not clear if all of those cases are linked to eggs, however.

Salmonella is the most common bacterial form of food poisoning. And the strain involved in the outbreak is the most common strain of salmonella, accounting for roughly 20 percent of all salmonella food poisonings.

Unfortunately, current lab tests do not allow health officials to fingerprint this form of salmonella as precisely as other kinds of food-borne illness. So determining the size of a salmonella enteritidis outbreak is a little more challenging, Braden said.

Much of the investigation so far has been centered on restaurants in California, Colorado and Minnesota where sick people ate. They are not necessarily breakfast places — it's possible some got sick from eating a salad dressing that had a raw egg in it, or eating soup with an undercooked egg dropped in, Braden said.

In one state, a cluster of illnesses seem to stem from meringue pie, health officials said.

The eggs were distributed around the country and packaged under the names Lucerne, Albertson, Mountain Dairy, Ralph's, Boomsma's, Sunshine, Hillandale, Trafficanda, Farm Fresh, Shoreland, Lund, Dutch Farms and Kemp.

The initial recall was issued last week. Eggs affected by the expanded recall were distributed to food wholesalers, distribution centers and food service companies in California, Colorado, Minnesota, Arkansas, Arizona, Georgia, Illinois, Iowa, Oklahoma, Oregon, Missouri, Nebraska, Nevada, Texas, Utah, Washington and Wisconsin.

"We are undertaking this additional recall to further protect the safety of consumers — this voluntary measure is consistent with our commitment to egg safety, and it is our responsibility," Wright County Egg officials said in a statement Wednesday evening.

In an earlier statement, company officials said the FDA is "on-site to review records and inspect our barns." The officials said they began the recall Aug. 13.

The recall is causing concern to consumers that other foods may contain contaminated eggs. In a statment to FoxNews.com, Kraft Foods, who also own Nabisco, said "We do not purchase eggs from this supplier so there is no impact to Kraft Foods."

The most common symptoms of salmonella are diarrhea, abdominal cramps and fever within eight hours to 72 hours of eating a contaminated product. It can be life-threatening, especially to those with weakened immune systems.

This form of salmonella can be passed from chickens that appear healthy. And it grows inside eggs, not just on the shell, Braden noted. The fate of the chickens will be determined by Wright County Egg after the FDA completes the investigation, according to Elaine Gansz Bobo, a FDA spokeswoman.

Thoroughly cooking eggs can kill the bacteria. But health officials are recommending people throw away or return the recalled eggs.

The Associated Press and Wall Street Journal contributed to this article.