The officials said investigators found salmonella in chicken feed that was sold to both Wright County Egg and Hillandale Farms. They also found salmonella samples at Wright County Egg.
More than 550 million eggs from the two farms were recalled this month after they were linked to more than a thousand of cases of salmonella poisoning.
Egg Recall: Complete Coverage
Sherri McGarry of the FDA's Center for Food Safety and Applied Nutrition said the tests indicate that contaminated feed is a source of the outbreak but possibly not the only source.
An official with the Centers for Disease Control and Prevention said between May 1 and August 25, a total of 2,403 illnesses were reported. However, he said "some cases from this time period have not been reported yet, and some of these cases may not be related to this outbreak."
"This outbreak represents the largest salmonella enteriditis outbreak since the start of outbreak surveillance in the mid-1970s," Dr. Christopher Braden, acting director of the division of foodborne, waterborne and environmental diseases at the CDC said. "The next largest one was in 1994 due to a commercial ice cream product that was contaminated by raw eggs."
Thoroughly cooking eggs can kill the bacteria. But health officials are recommending people throw away or return the recalled eggs.
The House Energy and Commerce Committee will hold hearings on the recall in September.
The Associated Press contributed to this report.