The boss of the US peanut processor at the centre of the country's salmonella outbreak falsified documents and pressured regulators as he riled against the cost and delay the contamination was causing his company, internal documents obtained by Congress reveal.

Documents released by the House Committee on Energy and Commerce yesterday (11 February) showed that Stewart Parnell, president of Peanut Corporation of America, pushed federal regulators to permit him to continue using peanuts from the tainted plant in Blakely, Georgia.

According to internal memos and emails, Parnell also shipped products to customers with a homemade certificate that was misleadingly used as evidence of purity.

The email records also demonstrated that Parnell repeatedly tried to skirt internal tests that came back positive for salmonella by sending the samples to different laboratories for further testing.

When confronted with a positive result for salmonella in August, Parnell sent the sample to a new lab for retesting. When a negative result came back, Parnell wrote: "Okay, let's turn them loose then."

PCA is the peanut butter manufacturer at the centre of a massive food contamination scandal and is facing a federal criminal investigation.

Parnell was forced to appear at the Congressional hearing by a subpoena. However, he pleaded the Fifth Amendment, which gave him the right to not say anything incriminating, and answered no questions.

The outbreak has sickened at least 600 people and has been linked to eight deaths.