OMAHA, Neb. (AP) -- The Food and Drug Administration said Friday it will increase the frequency of investigations at plants that make peanut butter and similar products, saying this year's salmonella outbreak showed peanut butter is riskier than health officials had thought.

All Peter Pan and Great Value peanut butter that ConAgra Foods Inc. made at its Sylvester, Ga., plant was recalled in February after health officials linked the product to a salmonella outbreak that has sickened more than 400 people nationwide.

"Up until this point, peanut butter has not been considered a high-risk food," said Dr. David Acheson, chief medical officer of the FDA's Center for Food Safety and Applied Nutrition. "We now know peanut butter can be a vehicle for salmonella."

Acheson said peanut butter will almost certainly move up on the FDA's list of high-risk foods, and the agency bases its inspection schedule on the relative risk of foods. He said peanut butter is not likely to knock fresh produce off the top of that list, because the risks are highest with foods that don't get cooked later.

Acheson said salmonella infections could happen at other peanut butter plants, but he believes the industry has been paying attention.

"I would be pretty certain that every other peanut butter producer is having the same thought we are and is paying a lot of attention to it to make sure that it doesn't happen."

The explanation for the salmonella outbreak ConAgra officials offered Thursday fits with what the FDA found, Acheson said, but the government investigation has not been completed.

FDA officials will decide whether to pursue any sanctions against ConAgra after the investigation, Acheson said.

ConAgra spokeswoman Stephanie Childs said Thursday the company traced the salmonella outbreak to three problems at its Sylvester, Ga., plant last August.

The plant's roof leaked during a rainstorm, and the sprinkler system went off twice because of a faulty sprinkler, which was repaired.

The moisture from those three events mixed with dormant salmonella bacteria in the plant that Childs said likely came from raw peanuts and peanut dust.

The plant was cleaned thoroughly after the roof leak and sprinkler problem, but the salmonella remained and somehow came in contact with peanut butter before it was packaged, she said.

The FDA last inspected the ConAgra plant in 2005 and did not find any problems.

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