The U.S. Food and Drug Administration is trying to find the cause of a salmonella outbreak that has made as many as 1,300 people ill and forced the recall of more than a half-billion eggs.

A warning sign about a possible food-borne illness is posted at a Los Angeles grocery store. (Damian Dovarganes/Associated Press)

Two Iowa farms linked to the disease outbreak — Wright County Egg and Hillandale Farms — share suppliers of chickens and feed as well as ties to an Iowa business with a history of violating state and federal law.

The number of illnesses, which can be life-threatening, especially to those with weakened immune systems, is expected to increase. The most common symptoms are diarrhea, abdominal cramps and fever eight to 72 hours of eating a contaminated product.

Jewanna Porter, a spokeswoman for the egg industry, said Saturday the company Quality Egg supplies young chickens and feed to both Wright County Egg and Hillandale Farms. The two share other suppliers, she said, but she did not name them.

The egg industry has consolidated over recent years, placing fewer, larger businesses in control over much of the nation's egg supply to consumers.

The salmonella outbreak has raised questions about federal inspections of egg farms. The FDA oversees inspections of shell eggs, while the Agriculture Department is in charge of inspecting other egg products.

Wright County Egg on Highway 69, near Galt, Iowa. After an outbreak of salmonella in several states, investigators traced the problem to Wright County Egg, leading to a recall of 380 million eggs. It's one of the largest shell egg recalls in recent history. (Nirmalnedu Majumdar/Associated Press)

William D. Marler, a Seattle attorney for a person who filed suit alleging illness from tainted eggs in a salad at a restaurant in Kenosha, Wis., said Sunday his firm has been retained by two dozen families and was representing a woman who was hospitalized in California.

Businessman Austin (Jack) DeCoster owns Wright County Egg and Quality Egg. Wright County Egg recalled 380 million eggs Aug. 13 after it was linked to more than 1,000 cases of salmonella poisoning. A week later, Hillandale Farms recalled 170 million eggs.

DeCoster's companies have a long history of problems:

• In 1994, the state of Iowa assessed at least four separate penalties against DeCoster Farms for environmental violations, many of them involving hog waste.
• In 1997, DeCoster Egg Farms agreed to pay $2 million in fines to settle citations brought in 1996 for health and safety violations at DeCoster's farm in Turner, Maine. The nation's labour secretary at the time, Robert Reich, said conditions were "as dangerous and oppressive as any sweatshop." Reich's successor, Alexis Herman, called the state of the farms "simply atrocious," citing unguarded machinery, electrical hazards, exposure to harmful bacteria and other unsanitary conditions.
• In 2000, Iowa designated DeCoster a "habitual violator" of environmental regulations for problems that included hog manure runoff into waterways. The label made him subject to increased penalties and prohibited him from building new farms.
• In 2002, the federal Equal Employment Opportunity Commission announced a more than $1.5-million settlement of an employment discrimination lawsuit against DeCoster Farms on behalf of Mexican women who reported they were subjected to sexual harassment, including rape, abuse and retaliation by some supervisory workers at DeCoster's Wright County plants.
• In 2007, 51 workers were arrested during an immigration raid at six DeCoster egg farms. His farms had been the subject of at least three previous raids.
• In June 2010, Maine Contract Farming, the successor company to DeCoster Egg Farms, agreed in state court to pay $25,000 in penalties and to make a one-time payment of $100,000 to the Maine Department of Agriculture over animal cruelty allegations that were spurred by a hidden-camera investigation by an animal welfare organization.

In a statement Sunday, Wright County Egg spokeswoman Hinda Mitchell said the company had reacted quickly in the past to correct its operations when "issues have been raised about our farms."

"We are approaching our work with FDA in the same forthright manner," she said.

The FDA investigation could take months, and sources of contamination are often difficult to find.

The CDC said last week that investigations by 10 states since April have identified 26 cases where more than one person became ill. Preliminary information showed that Wright was the supplier in at least 15 of those cases.