No deaths have been confirmed, although a Pennsylvania family filed a lawsuit Wednesday claiming a relative died from eating tainted peanut butter.
Opened jars from people who were sickened in New York, Oklahoma and Iowa tested positive for salmonella, said Dave Daigle, a spokesman for the CDC in Atlanta.
"Now the question becomes, how did the salmonella get in the jar," Daigle said.
ConAgra last week recalled all Peter Pan and Great Value peanut butter made at its Sylvester, Ga., plant after federal health officials linked the product to a salmonella outbreak that has sickened at least 329 people from 41 states since August.
Leslea Bennett-Webb, spokeswoman for the Oklahoma Department of Health, said the state recovered seven peanut butter jars from 11 cases confirmed by the state and found the strain of salmonella in at least one.
In Iowa, Kevin Teale, spokesman for the state's Department of Health, said the positive match is from one of the state's six confirmed cases.
At least 51 people were hospitalized with symptoms of the disease between Aug. 1 and Feb. 2, with 60 percent of illnesses beginning after Dec. 1, according to the CDC.
Salmonella, which commonly originates from the feces of birds and animals, 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.
ConAgra learned of the test results Thursday, spokesman Chris Kircher said.
Gary Rodkin, chief executive of Omaha-based ConAgra, said Thursday that the company will take "all reasonable steps to remedy the situation."
"We are truly sorry for any harm that our peanut butter products may have caused," Rodkin said in a news release.
Government and industry officials have said the contamination may have been caused by dirty jars or equipment. Peanuts are usually heated to high, germ-killing temperatures during the manufacturing process. The only known salmonella outbreak in peanut butter — in Australia during the mid-1990s — was blamed on unsanitary plant conditions.
ConAgra has said none of its previous routine testing of plant equipment and peanut butter has tested positive for salmonella. The Food and Drug Administration last inspected the plant in February 2005 and found no problems.
The Sylvester plant is the sole maker of the nationally distributed Peter Pan brand, and the recall covers all peanut butter produced by the plant since May 2006. Shoppers are being asked to toss out jars having a product code on the lid beginning with "2111," which denotes the plant. The jars or their lids can be returned to the store where they were purchased for a refund.
Great Value peanut butter is a Wal-Mart Stores Inc. house brand made by several manufacturers. Great Value peanut butter that does not have the "2111" code is not included in the recall.