Chicken feed contaminated with salmonella bacteria could have caused the outbreak at two Iowa producers that sparked a recall of more than a half billion contaminated eggs last week, U.S. regulators said on Thursday.

We've received confirmation of salmonella-positive ... with the DNA fingerprint that matches the outbreak fingerprint for the feed samples that were provided to pullet at the Wright County egg farms at Hillandale Farms. said Sherri McGarri, foodborne outbreak coordinator at the U.S. Food and Drug Administration.

The feed linked to the outbreak was produced at a feed mill that is part of Wright County Egg operation that recalled 380 million eggs last week. The second farm, Hillandale Farms, also received that feed, McGarri said.

She said 600 samples were collected from 24 possible sources on the two Iowa farms. While the feed was the likely source of contamination, it may not be the only one and more samples will be analyzed, McGarri said.

Samples that tested positive for salmonella were collected from barns at the two farms, and from feed ingredients given to pullets at both farms.

The outbreak was linked to almost 2,400 cases of salmonella-related illnesses around the country since May 1, in the largest outbreak since the 1970.

The egg recalls came weeks after a new FDA rule took effect requiring large-scale producers to practice better safety and to test in the poultry house for salmonella bacteria.