ATLANTA -- ConAgra Foods Inc. told consumers to discard certain jars of Peter Pan and Great Value peanut butter after the spread was linked to a salmonella outbreak that has sickened almost 300 people nationwide.

Lids of jars with a product code beginning "2111" can be returned to ConAgra for a refund, the company said.

The salmonella outbreak, which federal health officials said Wednesday has sickened 288 people in 39 states since August, was linked to tainted peanut butter produced by ConAgra at a plant in Sylvester, Ga. How salmonella got into peanut butter is still under investigation, said Dr. Mike Lynch, an epidemiologist at the Centers for Disease Control and Prevention.

CDC officials believe the salmonella outbreak to be the nation's first stemming from peanut butter. The most 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, Lynch said. About 85 percent of the infected people said they ate peanut butter, CDC officials said.

ConAgra officials said it was unsure why the CDC identified peanut butter as the source of the problem. Its own tests of its peanut butter and the plant have been negative, but it shut down the plant so it can investigate, spokesman Chris Kircher said.

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

Kircher called the recall a precaution. "We want to do what's right by the consumer," he said.

ConAgra officials haven't said how much peanut butter is covered in the recall. The Peter Pan brand is sold in 10 varieties, according to ConAgra's Web site. The Great Value brand, which is also made by other companies, is a Wal-Mart brand.

He said the CDC contacted the Food and Drug Administration, which sent investigators to the Georgia plant to review records, collect product samples and conduct tests for salmonella.

Kircher said ConAgra makes peanut butter only at the Sylvester plant, for distribution nationwide.

ConAgra randomly tests 60 to 80 jars of peanut butter that come off the line each day for salmonella and other pathogens, he said.

"We've had no positive hits on that going back for years," Kircher said.

The plant itself is also regularly tested, he said, though he didn't know how often. He said none of those tests have detected salmonella either.

The latest outbreak began in August, with no more than two cases reported each day, CDC officials said. Only in the past few days did investigators hone in on peanut butter as a source, Lynch 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.

Salmonella infection is known each year to sicken about 40,000 people in the United States, according to the CDC. Salmonellosis, as the infection is known, kills about 600 people annually.

Symptoms can include diarrhea, fever, dehydration, abdominal pain and vomiting.

The recall does not affect Great Value peanut butter made by other manufacturers, the FDA said.

Shares of ConAgra stock rose 13 cents to $25.98 in morning trading on the New York Stock Exchange.