Mr. DeCoster, who is known as Jack and is in his 70s, and his son, Peter, who many here say now largely oversees the Iowa operation, declined through a spokeswoman to be interviewed for this article. The spokeswoman, Hinda Mitchell, also declined to answer questions about earlier environmental and labor complaints against the DeCoster operation or about the DeCosters themselves. Of this Iowa region’s relations with the DeCosters, Ms. Mitchell said, “The farms have been longtime, active members of our local community.”

The precise cause of the salmonella infections has not been pinpointed, but Thursday’s revelations by federal authorities suggested they were moving closer.

Sherri McGarry, a Food and Drug Administration official, said salmonella was found in feed given to young birds, known as pullets, that were raised by a DeCoster facility for use at both its own farms and at Hillandale Farms. The bacteria was also found in bone meal, an ingredient used by the DeCoster operation to make its feed. In a statement, Wright County Egg suggested the contaminated ingredients could have come from a supplier.

The DeCosters have five large egg-laying facilities and eight sites for pullets.

Federal officials said the discovery was significant but that it did not mean that the contamination of the eggs  which occurs when they form inside infected hens  began with bad feed. Instead the salmonella could have been spread to the feed by rodents, workers or in other ways.

Mr. DeCoster came here from Turner, Me., a tiny town where he began with a paltry 125 hens and built an empire  crossing some people along the way. Robert B. Reich, then the labor secretary, once publicly denounced the DeCoster Egg Farms in Maine as an “agricultural sweatshop” where the workers were treated like “animals.”

But Larry Olson, who was a Wright County, Iowa, supervisor for 32 years, said he believed that the DeCosters had improved their practices in recent years. State environmental officials noted that Mr. DeCoster’s “habitual violator” status had expired, and that the Department of Natural Resources had not cited it for serious infractions in years.