The criminal division of the Food and Drug Administration and the Justice Department have joined the probe of the Iowa farm at the heart of the recent egg recall linked to an outbreak of salmonella, according to FDA Commissioner Margaret Hamburg.

"There is a formal investigation going on that extends beyond the FDA inspections that are focused on farm practice," Dr. Hamburg told reporters Wednesday. "It is the case that an investigation is under way. We are pursuing it with our partners in law enforcement."

Dr. Hamburg declined to discuss details or to say whether Federal Bureau of Investigation agents have visited facilities of Wright County Egg, a major egg producer that recalled 380 million eggs in mid-August.

FDA spokeswoman Pat El-Hinnawy said Wednesday that federal agents visited both Wright County Egg and Hillandale Farms, the second producer involved in the recall, on Tuesday. She referred further questions to the U.S. attorney in Cedar Rapids, Iowa, who couldn't immediately be reached for comment.

Wright spokeswoman Hinda Mitchell said FDA enforcement agents were at Wright on Tuesday. "Wright County Egg is cooperating fully," she said. A Justice Department spokeswoman declined to comment.