Two farms in Iowa are at the center of the nationwide recall of more than 500 million eggs.

More than a dozen government inspectors from the Food and Drug Administration are at the two farms operated by the Wright County Egg and Hillandale Farms, searching for the source of any salmonella contamination. Officials say the could be there until next week.

Both farms are associated with Jack DeCoster and the Iowa Supreme Court said his operations have a history of health and safety violations.

"In one situation, we had three individuals who became ill who all had been at the same catered breakfast and all had omelets," said Dr. Jonathan Fielding, director for the Los Angeles County Department of Public Health. "We traced it back to this Wright County egg in Iowa."

The number of illnesses, which can be life-threatening, especially to those with weakened immune systems, is expected to increase. The federal Centers for Disease Control has said there could be as many as 1,300 salmonella illnesses linked to the eggs. The CDC said that for every case reported, there could be 30 or more unreported cases.

One member of Congress has asked the Food and Drug Administration and U.S. Department of Agriculture to explain what they knew about the egg businesses before the recall.

New federal rules requiring more testing in hen houses could prevent similar outbreaks.

"It will require egg producers to do some monitoring of their flocks and their environment for salmonella but you still need to do much more to make the egg supply completely safe," said William Hubbard, a former FDA associate commissioner.

For more on salmonella poisoning, visit the Centers for Disease Control Web site