WASHINGTON — 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, an egg industry spokeswoman said.

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 after 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.