WASHINGTON (AP) – Federal regulators say they have found positive samples of salmonella that link two farms in the Midwest state of Iowa to a massive egg recall.

Investigators found salmonella in chicken feed at Wright County Egg that was used by that farm and also Hillandale Farms, the Food and Drug Administration said.

Authorities also found additional samples of salmonella in other locations at Wright County Egg. 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, which raises new questions as to whether other egg farms also could have received contaminated feed. The FDA could not confirm the feed element came from a third party, but so far has said it does not believe the salmonella outbreak will expand beyond the two farms.

Also Thursday, the federal Centers for Disease Control and Prevention said there could now be as many as 1,470 illnesses linked to the outbreak, about 200 more than previously thought.

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.

McGarry and other FDA officials emphasized in a briefing for reporters 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.

Wright County Egg said one of the feed ingredients from Central Bi-Products was held separately in a bin that was tested by FDA officials.

Wright County Egg spokeswoman Hinda Mitchell said Central Bi-Products has no ties to the two farms, and they receive the feed ingredient, which contains meat and bone meal, from a distributor.

"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 the Wright County Egg statement attributed to unidentified company officials.