The Food and Drug Administration commissioner on Monday said there may be more recalls of eggs in the salmonella outbreak and the agency did not yet know how the eggs and chickens were contaminated.

We don't know exactly how the contamination got into the chicken population, into the egg population, and we're not yet fully sure the extent of the recall that will be necessary to protect consumers, FDA Commissioner Margaret Hamburg said on ABC's Good Morning America.

We are in the midst of probably the largest egg recall that has happened in recent history, she said.

Her comments came after federal regulators on Friday said a second Iowa egg farm, Hillandale Farms of Iowa, voluntarily recalled eggs as part of a U.S. salmonella outbreak that is linked to almost 300 illnesses across the United States.

The other potential source is Iowa egg producer Wright County Egg, which recalled 380 million eggs last week.

We're continuing to investigate aggressively to determine the exact source of the contamination, Hamburg said on NBC's Today show.

As we move forward with the recall, we may see some additional sub-recalls over the next couple of days, maybe even weeks as we better understand the sort of network of distribution of these eggs that are potentially contaminated, she said.

Salmonella can cause fever, diarrhea, vomiting and abdominal pain and sometimes more serious illness or death.

Hamburg's advice for consumers was to prepare food properly, keep the eggs refrigerated, wash hands before and after handling eggs and cook the egg thoroughly.

No more runny egg yolks for mopping up with toast, she said.