WASHINGTON (AP) — Two Iowa farms that recalled more than a half-billion eggs linked to as many as 1,300 cases of salmonella poisoning share suppliers of chickens and feed as well as ties to an Iowa business routinely cited for violating state and federal law.

Food and Drug Administration investigators have yet to determine the cause of the salmonella outbreaks at Wright County Egg and Hillandale Farms. The FDA investigation could take months, and sources of contamination are often difficult to find.

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.

The company Quality Egg supplies young chickens and feed to both Wright County Egg and Hillandale Farms. The two share other suppliers, said Jewanna Porter, a spokeswoman for the egg industry, 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.

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.

“The history of ignoring the law makes the sickening of 1,300 and the forced recall of 550 million eggs shockingly understandable,” Marler said in an e-mail to The Associated Press. “You have to wonder where the USDA and FDA inspectors were.”

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 is no stranger to controversy in his food and farm operations:

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 labor 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 issued Sunday, Wright County Egg spokeswoman Hinda Mitchell said: “When issues have been raised about our farms, our management team has addressed them swiftly and effectively, working with recognized outside experts to identify and establish corrective measures for our operations. We are approaching our work with FDA in the same forthright manner.”

Wright County Egg also faces a lawsuit from food distributor Dutch Farms alleging that the company used unauthorized cartons to package and sell eggs under its brand without its knowledge.

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.