U.S. health officials have opened a criminal investigation into the actions of the Peanut Corporation of America, which they said they were aware that they had knowingly sold tainted peanut butter and peanut items to major food makers.

The investigation is being conducted by the Food and Drug Administration and the Justice Department, says Dr. Stephen F. Sundlof, director of the food center at the Food and Drug Administration.

The agency almost never discloses criminal investigations that are presently in progress, but did so in this situation after a number of influential members of Congress requested a criminal investigation in recent days.

The tainting of the company’s foods with salmonella poisoning has started one of the most significant food recalls in national history. More than 400 products containing peanuts, peanut butter or peanut meal have been recalled, and more recalls are anticipated.

On Monday, the company increased its recall to include items made at its plant in Blakely, Ga., in 2007 and the first 6 months of 2008; it had previously recalled all products produced there from July forward.

The Food and Drug Administration also disclosed that in April it denied to allow a shipment of peanut items from the company to be brought back into America from Canada because it contained metal content. The company attempted to remove the shavings from the shipment but failed, and the items were discarded in November, the agency said.

As a result of that predicament, the agency asked state inspectors in Georgia to visit the plant to ensure that no more metal content would find their way into the items.

The April incident involving the denied shipment was apparently the first time the Food and Drug Administration was aware that the company was making peanut products, rather than just peanut butter, at its Blakely plant.

Agency officers said that name-brand peanut butter had proved safe to consume, but that some boutique brands of peanut butter for instance, those made in small batches at stores could be suspect.

The peanut tainting has now struck 529 people and is associated with eight deaths. Cases are still being reported to federal health officials, although the number of new cases has gone down quite a bit, said Dr. Robert Tauxe of the Centers for Disease Control and Prevention.

The White House press secretary, Robert Gibbs, said Friday, I think the revelations have no doubt been surprising, that whether it was our own regulatory system or a company that continuously found salmonella in its own testing would continue to deliver that product is beyond disturbing for millions of parents.