(CNN) -- The maker of peanut butter linked to a nationwide outbreak of salmonella shipped tainted product it knew had tested positive for the bacteria, the Food and Drug Administration said Wednesday.

The Peanut Corporation of America found salmonella in its plant in Blakely, Georgia, the FDA said.

The FDA report said the Peanut Corporation of America's own testing program found strains of salmonella 12 times in 2007 and 2008 at its Blakely, Georgia, plant. The problem does not appear to have been resolved.

When FDA inspectors visited the plant this month, they reported finding still more salmonella contamination.

According to the inspection report, posted on the FDA's Web site, the "firm's own internal microbiological testing" found salmonella in peanut paste, peanut butter, peanut meal, peanut granules and oil-roasted, salted peanuts. Watch what violations occurred at the plant »

However, it added, "After the firm retested the product and received a negative status, the product was shipped."

That's not the way it ought to have been handled, according to one expert. "They were lab shopping," said Tommy Irvin, Georgia's agriculture commissioner. "They were trying to find a way to clear their product, so they can ship their product out," he told CNN. Learn more about food poisoning »

He said proper practices demand that if any food product tests positive for salmonella and another test comes back negative, "you believe the one that is positive."

In a written statement, the company denied accusations it had been "lab shopping" to get a negative test result in order to ship the product.

"PCA uses only two highly reputable labs for product testing and they are widely used by the industry and employ good laboratory practices," the company said. "PCA categorically denies any allegations that the company sought favorable results from any lab in order to ship its products."

But according to Irvin, once salmonella is found in a product, "that lot should be destroyed, but [in this case it] wasn't."

The Georgia Department of Agriculture is working with the FDA on the investigation of the outbreak, which has been linked to the plant.

"The inspection also revealed no steps were taken in terms of cleaning or cross-contamination" after the salmonella was found in the plant, said FDA's director of the Center for Food Safety and Applied Nutrition, Dr. Stephen Sundlof.

The company did not clean the production line after Salmonella Typhimurium, the bacterium implicated in the outbreak, was found there last September, according to the FDA report. This is the same type of bacteria found in 502 people who have become ill in 43 states and Canada since September. At least eight deaths have been linked to the outbreak.

Violations also include contamination of plant surfaces and equipment by other microorganisms, the discovery of roaches near production and packaging areas and the inability of the company's ventilation system to prevent the salmonella from contaminating other parts of the plant.

Sundlof said the reported problems indicate the plant deviated from the good manufacturing practices companies are supposed to follow.

The FDA investigation began January 9, shortly after the manufacturer was implicated as a source of the outbreak. The plant produces peanut butter sold to institutions, such as nursing homes and cafeterias, as well as peanut paste, which is used in cookies, crackers, ice cream and pet treats. Watch the salmonella outbreak that has sickened hundreds »

This month, Peanut Corporation of America began recalling peanut butter and paste produced since July 1. The recall was expanded Wednesday to include products produced since January 1, 2007.

More than 300 products using PCA's peanut paste and peanut butter have already been recalled and the FDA has urged consumers to check the agency's Web site frequently for updates. See a list of recalled products

Federal health officials recommend that consumers throw away any recalled products and not consume any products whose safety cannot be verified.

The American Peanut Council has compiled a list of companies not implicated in the recall on www.peanutsusa.com.

CNN's Saundra Young contributed to this report