The outbreak was traced back to a factory in Georgia
A peanut supplier blamed for a US-wide outbreak of salmonella poisoning sold nuts it knew were infected, the Food and Drug Administration (FDA) says.
The FDA was revising an earlier report it had issued saying the Georgia-based firm Peanut Corporation of America (PCA) had waited for negative results.
A criminal inquiry was launched last month. The corporation denies any wrongdoing and says it is co-operating.
The outbreak has been linked to eight deaths and more than 500 illnesses.
The FDA carried out an inspection of the corporation's plant in the town of Blakely in January, after it was identified as a possible source of the contamination.
The inspection found that the factory was "not compliant with Current Good Manufacturing Practices required by the FDA".
The FDA's initial report had stated that all of the batches of nuts that had tested positive for salmonella had been retested by the firm, and negative results had been returned before the products were sold.
But in a statement, the administration said: "Certain information provided by PCA management during the inspection was not consistent with the subsequent analysis of the company's records."
In its new report, the FDA details several instances going back to 2007 when the firm shipped out products that had tested positive for salmonella before receiving the results of second tests.
In some cases, the FDA said, the firm received a subsequent negative test, and in other cases "no additional testing appears to have been done".
Salmonella causes diarrhoea, vomiting and fever
More than 1,300 food products containing PCA peanut butter have now been recalled.
A total of 575 people in 43 US states are reported to have fallen ill with the outbreak strain of salmonella typhimurium since September 2008.
More than half of those affected were under 16 years old.
Eight deaths have also been reported, which the FDA said earlier "appeared to be associated with the outbreak".
PCA announced a voluntary recall of its peanut butter on 13 January and has apologised "to peanut consumers everywhere" for the crisis.
The company has not denied that its plant was the source of contamination.
Earlier, officials said national brands of peanut butter have not been affected, but they have warned consumers to be cautious with peanut products produced by smaller companies.
Symptoms of salmonella poisoning, which usually appear within 12 to 72 hours of eating contaminated food, include diarrhoea, nausea and vomiting, headaches, stomach cramps and fever.
The symptoms can last for several days but most otherwise healthy people make a full recovery within a week.