Food and Drug Administration officials said Thursday their investigators had homed in on chicken feed as a likely major contributor to the salmonella contamination that triggered a nationwide egg recall and potentially caused nearly 1,500 cases of illness.

Feed found at Wright County Egg in Iowa tested positive for salmonella, FDA officials said at a joint news conference with the Centers for Disease Control and Prevention. Salmonella also was found in walkways and manure at Wright County Egg, as well as in ingredients used in the feed. The samples of the salmonella were a genetic match to the salmonella that has made many people sick, officials said.

The feed also was given to young hens, called pullets, that were supplied to Hillandale Farms, also in Iowa.

“The finding of positive feed raises a lot of additional questions for us to answer,” said Dr. Jeff Farrar, the FDA’s associate commissioner for food protection.

Investigators took about 600 samples from 24 locations as part of their investigation of the two farms, which produced more than 550 million eggs that were recalled after a spike in salmonella cases.

In addition to the chicken feed, other potential sources of contamination may emerge because the testing has not been completed, said Sherri McGarry, an emergency coordinator for the FDA.

The salmonella samples found in the feed could signal a larger problem of site contamination, said Dr. Joshua Sharfstein, FDA deputy commissioner.

“There is evidence of contamination at the farm. While they have found it in feed, they are not concluding any cause-and-effect relationship,” he said. “There are multiple potential routes of contamination.”

The feed undergoes a heat treatment that kills salmonella, suggesting it may have been contaminated at the farms after it was processed. FDA officials believe Wright County Egg and Hillandale were the only farms affected by the feed.

Officials said that they did not expect the egg recall to expand to other farms, but that more illnesses are likely to be reported and smaller recalls for additional brands may be announced.

In a statement, Hillandale Farms said it and Wright County Egg “share a number of common suppliers because they are in the same industry in the same state. One of those suppliers is Quality Egg, LLC which supplies pullets and feed to both companies and is owned by the [Jack] DeCoster family. Hillandale Farms does not discuss the specifics of supplier relationships because that is business confidential information.”

The CDC noticed an increase in the number of salmonella cases in late May. By late July, the center reported that states had identified 26 “distinct clusters” of salmonella infection. FDA officials held off on notifying the public of the outbreak because they did not have definitive lab results until Aug. 11, Farrar said.

“We have to strike the balance of being timely and being accurate, and that is what we do in every investigation,” Farrar said. “We would be the first to admit that there is always room for improvement.”

Farrar noted that legislation pending in Congress to improve the traceability of food products, ease access to records and give the FDA the authority to order recalls would have sped up the egg recall.

Since the recall was announced Aug. 13, Wright County Egg has been sending those eggs to its egg-breaking plant on its site. It’s also sending eggs to other egg-breaking plants and pasteurization facilities across the Midwest. Pasteurization kills any salmonella in the eggs.

“The liquid eggs are being treated, which then is used in a variety of products” including cake mixes, pastas and other processed foods, said Wright County Egg spokeswoman Hinda Mitchell. “The process makes the eggs safe and follows federal regulations.”

julia.love@latimes.com

Times staff writer P.J. Huffstutter in Los Angeles contributed to this report.