The U.S. Food and Drug Administration (FDA) has linked a recent outbreak of salmonella with tainted chicken feed that was used at two Iowa egg farms.

Investigators announced on Thursday that salmonella bacteria has been detected in chicken feed that was used at the farms.

Although FDA standards require chicken feed to be heated to a temperature sufficient to kill salmonella bacteria, investigators believe the feed was contaminated sometime after the heat treatment and before it was fed to the chickens.

Over 500 million eggs have been recalled.

During the outbreak, which spanned from May 1 and August 25, over 2,430 cases of salmonella were reported, according to the Centers for Disease Control and Prevention.

Salmonella bacteria can be fatal, although no deaths were reported during this outbreak.