Early tests suggested chicken feed may have become contaminated after it arrived at the Iowa farm at the center of a huge egg recall, helping investigators zero in on the cause of this summer’s salmonella outbreak.

The Food and Drug Administration said earlier that it found salmonella in feed at Wright County Egg Co. but didn’t know how the bacteria got there.

The agency has since collected and tested samples from companies that shipped feed ingredients to Wright County Egg, which had its own mill that produced finished feed for its chickens. Those tests so far have failed to find salmonella at the suppliers, raising the possibility that the contamination occurred after the feed shipments were delivered, FDA officials said.