A feed ingredient containing meat and bone meal fed to chickens on two Iowa farms may be the source of a salmonella outbreak in the U.S. that has sickened at least 1,470 people.

Investigators found salmonella in chicken feed used at Wright County Egg farm in Galt, Iowa, and at Hillandale Farms in New Hampton, Iowa, the Food and Drug Administration said.

Authorities also found additional samples of salmonella in other locations at Wright County Egg.

One of the chicken confinement sites operated by Wright County Egg in Galt, Iowa. A salmonella outbreak has been traced to a meat and bone meal feed used on the farm. (Ryan J. Foley/Associated Press)More than 550 million eggs from the two farms were recalled this month after they were linked to salmonella poisoning in several states.

One positive sample for salmonella was found in a feed ingredient sold to Wright County Egg from a third-party supplier, Central Bi-Products, according to the egg company. That raises new questions as to whether other egg farms also could have received contaminated feed.

Sherri McGarry of the FDA's Center for Food Safety and Applied Nutrition said the salmonella found at Wright County Egg matches the fingerprint of the salmonella found in many of those who were sickened. She said the tests indicate that contaminated feed is a source of the outbreak but possibly not the only source.

Investigation is ongoing

McGarry and other FDA officials emphasize that the agency's investigation is continuing, and they do not yet know how the feed became contaminated. Investigators are analyzing as many as 600 samples from 24 locations at the two farms.

"We will work with FDA as they expand their review of feed ingredients purchased from outside vendors for our farm, as well as for their ongoing review of our farms," said a Wright County Egg statement.

A sign that reads "Our eggs are all right!!!" hangs next to cartons of eggs that are ready for sale in Lawrence, Kan. A salmonella outbreak that has sickened 1,470 people has been traced to feed used on two Iowa farms. (Orlin Wagner/Associated Press)Wright County Egg's statement also asserted that a positive test does not mean eggs from that barn would have salmonella. Mitchell said the company had tested some eggs from one of the barns where salmonella was found since July, and those eggs did not test positive for salmonella.

Officials said they are not yet certain whether the feed came to the farms contaminated or was contaminated at the farms. They said there is no evidence at this time that the feed went to any other farms.

FDA Deputy Commissioner Dr. Joshua Sharfstein said investigators are not looking only at the feed but at the "overall contamination of the facility." Contamination found in the feed could be a part of a larger problem there, he said.

"While they have found it in the feed, they are not confirming any sort of cause and effect relationship," he said.

Salmonella can be life-threatening, especially to those with weakened immune systems, and the number of illnesses is expected to increase. No deaths have been reported as a result of the outbreak.

Dr. Christopher Braden, an epidemiologist with the Centers for Disease Control and Prevention, said this is the largest outbreak of this strain of salmonella since the start of the agency's surveillance of outbreaks in the late 1970s. The second largest was an outbreak caused by raw eggs in ice cream in the 1990s in which more than 700 people became ill.

Thoroughly cooking eggs can kill the bacteria. But health officials are recommending people throw away or return the recalled eggs.