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.

“We think there’s very strong evidence that it was this brand of peanut butter,” Lynch said.

Salmonella sickens about 40,000 people a year in the U.S. and kills about 600.