WASHINGTON — The Georgia peanut plant linked to a salmonella outbreak that has killed eight people and sickened 500 others across the country knowingly shipped out contaminated peanut butter 12 times in the past two years, federal officials said yesterday.

Officials at the Food and Drug Administration and the Centers for Disease Control and Prevention, which have been investigating the salmonella outbreak, said yesterday that the Peanut Corp. of America found salmonella in internal tests a dozen times in 2007 and 2008 but sold the products anyway, sometimes after getting a negative finding from a different laboratory.

Companies are not required to disclose their internal tests to either the FDA or state regulators, so health officials did not know of the problem.

The peanut butter and paste made at the company's Blakely, Ga., plant are not sold directly to stores but are used by manufacturers to make crackers, cookies, energy bars, cereal, ice cream, candies and even dog biscuits. Some of the country's biggest foodmakers, including Kellogg and McKee Foods, which produces Little Debbie brand snacks, have recalled more than 100 products made with the tainted ingredients, and the list keeps growing.

Federal investigators also said yesterday that they had found four strains of salmonella at the Georgia plant, including one in a sample taken from the floor near a washroom. Only one strain — salmonella Typhimurium — has been linked to the outbreak.

"There is a salmonella problem at the plant," said Robert Tauxe, deputy director of the CDC's division of foodborne, bacterial and mycotic diseases.

The outbreak, which has spread to 43 states, including Hawai'i, and Canada, is ongoing, but the pace has slowed "modestly," Tauxe said. Half the people made ill have been children.

Major-label peanut butter is not suspected to be contaminated with salmonella and is considered safe to eat, according to the FDA. The makers of several major brands, including Peter Pan, Jif and Smuckers, are worried that panicky consumers will stop buying their products, and they have been taking pains to point out that their peanut butters are not part of the outbreak.

Peanut Corp. of America, based in Lynchburg, Va., was not required under federal or state law to inform regulators about its internal salmonella tests. But Stephen Sundlof, director of the FDA's Center for Food Safety and Applied Nutrition, said it appears that the company violated federal law. "Foods are supposed to be produced under conditions that do not render them damaging to health," he said.

Sundlof said he could not say whether the company might face criminal charges.

Stewart Parnell, the company's owner and president, was not taking calls yesterday, and his spokesman did not return telephone messages. The company halted production at the Blakely plant once the FDA confirmed it was the source of the outbreak. FDA officials said the company is free to restart production but will have to first address a list of manufacturing deficiencies, which federal officials intend to make public today.

FDA officials said they still do not know the how the plant was contaminated and how the bacteria got into the peanut products, although state inspection records show a pattern of unsanitary conditions over several years. In each case, inspectors flagged the problems but said they required routine follow-up. There is no evidence that Peanut Corp. of America was ever closed by the state or otherwise penalized.