Investigators trace infection to chicken feed.
FDA investigators have confirmed that a strain of salmonella found in two barns at Iowa's Wright County Egg and Hillandale Farms is linked to outbreak that has sickened thousands.
They found the bacteria in several samples, including feed given to chickens at the two farms.
What they don't know yet is how the salmonella got there.
The companies have agreed not to sell eggs to the public until the investigation is over.
Instead they're selling them to facilities that crack open the eggs and pasteurize them, which kills salmonella.
Eggs from the farms that had been contaminated will likely end up in some kitchens in the form of cookies, ice cream and other processed food.