Minnesota disease investigators once again may have solved the riddle of a nation-wide salmonella outbreak. This time the culprit is peanut butter.

Officials from the state Department of Health said late Friday that the salmonella bacteria found in 30 Minnesotans believe to have been sickened by eating King Nut brand creamy peanut butter has the same genetic fingerprint as the salmonella bacteria found in 400 sick people in 42 states.

Further testing on cases outside Minnesota will be needed to confirm that the peanut butter is the source of those illnesses as well, "but we think it's likely," said department spokesman Doug Schultz.

The peanut butter, sold in 5-pound containers to food service companies that supply schools, hospitals and other institutions, does not usually end up on supermarket shelves.

Kirk Smith, supervisor of food-borne diseases at the state Department of Health, said the clue in this outbreak was that many of the Minnesotans who became ill had eaten in institutional settings. That included nursing homes, schools and colleges, he said.

"What they had in common was this brand of peanut butter," he said. "That was enough."

The peanut butter is not made in Minnesota, and attempts to reach company officials were unsuccessful.

Health officials say that the product should not be served, pending further instructions as the investigation progresses.

The report of peanut butter contamination comes almost two years after ConAgra recalled its Peter Pan brand peanut butter, which was eventually linked to at least 625 salmonella cases in 47 states.

Eating food contaminated with salmonella bacteria can result in abdominal cramping, diarrhea and fever. Anyone who believes they may have become ill as a result of eating this product or foods made with this product should contact their health-care provider, health officials said.

Last time it was jalapeños

This is the second time in less than a year that the state Department of Health has played a major role in solving a national disease outbreak. In July, state investigators traced the source of a mysterious salmonella outbreak that had stumped federal health officials for two months to jalapeño peppers. Initially, the Centers for Disease Control and Prevention had focused on tomatoes as the source of the outbreak that sickened more than 1,200 people in 43 states and Canada.

The department's sophisticated laboratory and its speedy approach to sleuthing has been credited with solving that and many other cases.

Part of its success is "Team Diarrhea," a team of University of Minnesota public health graduate students who assist in the multitude of interviews that such investigations require.

Last summer's outbreak and the government's initial inability to find the source of the infection exposed serious flaws in the U.S. food safety system, experts said.

Staff Writer Matt McKinney and the Associated Press contributed to this report. Josephine Marcotty • 612 673 7394