Wright County Egg on U.S. Highway 69, near Galt, is seen Friday, after an outbreak of salmonella in several states. Investigators traced the problem to Wright County Egg and Hillandale Farms in New Hampton, leading to a recall of a total of 550 million eggs. It's one of the largest shell egg recalls in recent history.

DES?MOINES (AP) -Two large Iowa farms have recalled 550 million eggs because of possible contamination with salmonella. Investigators from the Food and Drug Administration are trying to find the cause of the outbreak, but so far haven’t pinpointed the source.

The two Iowa sites, Hillandale Farms in New Hampton and Wright County Farms in Clarion, are both owned by Austin “Jack”?DeCoster.

Eggs originally announced as affected included the following brand names Lucerne, Albertson, Mountain Dairy, Ralph’s, Boomsma’s, Sunshine, Hillandale, Trafficanda, Farm Fresh, Shoreland, Lund, Dutch Farms and Kemps. Eggs are packed in varying sizes of cartons with Julian dates ranging from 136 to 225 and plant numbers 1026, 1413 and 1946.

Eggs affected by the expanded recall announcement were marketed through Albertsons, Farm Fresh, James Farms, Glenview, Mountain Dairy, Ralph’s, Boomsma, Lund, Kemps and Pacific Coast. Eggs are packed in varying sizes of cartons with Julian dates ranging from 136 to 229 and plant numbers 1026, 1942 and 1946

Dates and codes can be found stamped on the end of the egg carton or printed on the case label.

Eggs sit in a bucket before being cooked at the Drake Diner, Tuesday in Des Moines. The egg recall hasn't affected this popular breakfast spot in downtown Des Moines.

Food and Drug Administration Commissioner Margaret Hamburg said there is no evidence that there are additional farms involved in a massive recall of more than half a billion eggs.

Officials also said Monday they do not expect the number of eggs recalled to grow based on what they know now.

The Centers for Disease Control and Prevention said it has not identified additional clusters of illness that would indicate the outbreak has spread beyond two Iowa farms. By Wednesday’s press time there was still no information available on determining the cause.

Also Monday, the House Energy and Commerce Committee said it is investigating the outbreak and sent letters to both farms asking for detailed information about company operations, communications with the government and what they knew when.

Hillandale said the eggs were distributed to grocery distribution centers, retail groceries and food service companies which service or are located in 14 states including Arkansas, California, Iowa, Illinois, Indiana, Kansas, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Texas and Wisconsin.

A food safety expert at Cornell University in Ithaca, N.Y., said the source of the outbreak could be rodents, shipments of contaminated hens, or tainted feed. Microbiology professor Patrick McDonough said he was not surprised to hear about two recalls involving different egg companies, because in other outbreaks there have also been multiple sources.

Both plants could have a rodent problem, or both plants could have gotten hens that were already infected, or feed that was contaminated.

“You need biosecurity of the hen house, you want a rodent control program and you want to have hens put into that environment that are salmonella free,” McDonough said.

The salmonella bacteria is not passed from hen to hen, but usually from rodent droppings to chickens, he added. This strain of bacteria is found inside a chicken’s ovaries, and gets inside an egg.

Sources for this article include Associated Press, Federal Drug Administration web site and web sites from both affected farms.