(CNN) -- Feed given to hens -- used at both Wright County Egg and Hillandale Farms -- is the likely source of contamination that led to a nationwide salmonella outbreak, federal officials said Thursday.
The feed and some feed ingredients are believed to have been contaminated and are the source of the salmonella, U.S. Food and Drug Administration officials said in a conference call.
It's possible the feed became contaminated after it went through heat treatment that was sufficient to kill salmonella, officials said.
An official for the U.S. Centers for Disease Control said the reported cases of salmonella enteriditis stood at 2,430 between May 1 and Aug. 25. That number is expected to rise.
CNN's Phil Gast contributed to this article