Atlanta - Government scientists struggled Thursday to pinpoint the source of the first U.S. salmonella outbreak linked to peanut butter, the kid favorite packed into millions of lunchboxes every day.

Nearly 300 people in 39 states have fallen ill since August, and federal health investigators said they strongly suspect Peter Pan peanut butter and certain batches of Wal-Mart's Great Value house brand - both manufactured by ConAgra Foods Inc.

Shoppers across the country were warned to throw out jars with a product code on the lid beginning with "2111," which denotes the plant where it was made.

How the dangerous germ got into the peanut butter was a mystery. But because peanuts are usually heated to high, germ-killing temperatures during the manufacturing process, government and industry officials said the contamination may have been caused by dirty jars or equipment.

"We think we have very strong evidence that this was the brand of peanut butter. Now it goes to the next step of going to the place where the peanut butter was made and focusing in on the testing," said Dr. Mike Lynch, an epidemiologist at the Centers for Disease Control and Prevention.

The suspect peanut butter was produced by ConAgra at its only peanut butter plant, in Sylvester, Ga., federal investigators said.

ConAgra said it is not clear how many jars are affected by the recall. But the plant is the sole producer of the nationally distributed Peter Pan brand, and the recall covers all peanut butter - smooth and chunky alike - produced by the plant from May 2006 until now.

"We're talking a lot of jars of peanut butter," said Dr. David Acheson, chief medical officer of the Food and Drug Administration's Center for Food Safety and Applied Nutrition.

FDA inspectors visited the now shut-down plant Wednesday and Thursday to try to pinpoint where the contamination could have happened. The FDA last inspected the plant in 2005. Testing was also being done on at least some the salmonella victims' peanut butter jars, but investigators said some may have already been discarded.

The highest number of cases were reported in New York, Pennsylvania, Virginia, Tennessee and Missouri. About 20 percent of all the ill were hospitalized, and there were no deaths, the CDC said.

About 85 percent of the infected people said they ate peanut butter, and about a quarter of them ate it at least once a day, the CDC's Lynch said. It was the only food that most of the patients had all recently eaten.

Salmonella sickens about 40,000 people a year in the U.S. and kills about 600. It can cause diarrhea, fever, dehydration, abdominal pain and vomiting.

But most cases of salmonella poisoning are caused by undercooked eggs and chicken. The only known salmonella outbreak in peanut butter - in Australia during the mid-1990s - was blamed on unsanitary plant conditions.

ConAgra spokesman Chris Kircher said the company randomly tests 60 to 80 jars of peanut butter that come off its Sylvester plant's line each day for salmonella and other germs, and "we've had no positive hits on that going back for years." But he said the plant was shut down as a precaution for further investigation.

"We're trying to understand what else we need to do or should be doing," Kircher said.

An estimated 974 million pounds of peanut butter are sold each year in the U.S., and peanut butter and jelly is the most popular sandwich among children. Peter Pan is one of the nation's top three brands, though well behind market leader Jif. Great Value peanut butter is also produced by some other manufacturers for Wal-Mart.

In a measure of peanut butter's popularity, ConAgra's hot line was swamped with so many calls after the recall was announced on Wednesday that many people got a busy signal. School officials in Houston confiscated students' sandwiches from home and replaced them with those made at schools. And in Georgia, a lawmaker representing one of the nation's biggest peanut-producing areas warned colleagues to throw out jars of peanut butter that he recently handed out.

The outbreak was detected by the CDC and state health agencies when they noticed spikes in the cases of people sickened by an unusual type of salmonella, starting in August. Once peanut butter emerged as a link, the CDC notified the FDA.

Salmonella commonly originates in the feces of birds and animals, and could be introduced at a multitude of stages in the peanut butter-making process. But many safeguards are in place.

While rodents and birds commonly get into peanut storage bins, germs are killed when raw peanuts are roasted. When making peanut butter, the nuts are again heated - above the salmonella-killing temperature of 165 degrees - as they are ground into a paste and mixed with other ingredients before being squirted into jars and quickly sealed.

"The heating process is sufficient to kill salmonella, should it be present," said Mike Doyle, director of the University of Georgia's Center for Food Safety, in the state that produces nearly half of the nation's peanuts.

Experts say the point in the process where salmonella could be introduced and survive would be as the product cools down, is placed in the jars and then sealed. At most plants, those steps take just minutes.

But "there is quite a lot that happens after that heat step ... before it's put in jars. So there's definitely an opportunity for contamination after the roasting," the FDA's Acheson said.

Acheson speculated a small, on-again, off-again source of contamination caused the outbreak, which would explain the relatively small number of illness. That "will make finding it in peanut butter difficult. But that's not going to stop us from looking," he said.

Other states reporting cases are Alaska, Alabama, Arkansas, Arizona, California, Colorado, Connecticut, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Massachusetts, Maryland, Maine, Michigan, Minnesota, Mississippi, Montana, North Carolina, Nebraska, New Jersey, New Mexico, Ohio, Oklahoma, Oregon, South Carolina, South Dakota, Texas, Vermont, Washington, Wisconsin and West Virginia.

The strain in this outbreak, Salmonella serotype Tennessee, is comparatively rare, as is salmonella contamination of peanut products, said Caroline Smith DeWaal, director of food safety at the Center for Science in the Public Interest.

It may have taken a long time to identify peanut butter as the source because "it's just not one of the first things you'd suspect," Smith DeWaal said.