A spokeswoman for the U.S. attorney's office in Georgia, Pam Lightsey, said Tuesday that prosecutors plan to reveal details of the investigation Wednesday.
ConAgra spokeswoman Teresa Paulsen declined to comment on the investigation Tuesday, but the company previously has said it was negotiating an end to the investigation that would likely include a misdemeanour charge of shipping tainted products.
If the Omaha, Nebraska, food-maker is charged criminally, the case would extend a recent string of high-profile food safety prosecutions. It wasn't clear Tuesday whether any ConAgra executives would be charged.
Earlier this year, two former Iowa egg industry executives were sentenced to three months in jail. Last year, two Colorado cantaloupe farmers were convicted and received probation in a deadly 2011 listeria outbreak, and the former owner of Peanut Corporation of America was convicted in a 2008 salmonella outbreak. The peanut executive, Stewart Parnell, could face jail time when sentenced.
ConAgra recalled all its peanut butter in 2007 after its Peter Pan and Great Value peanut butter was linked to a salmonella outbreak that sickened at least 625 people in 47 states. The peanut butter was produced at ConAgra's Sylvester, Georgia, plant.
At the time of ConAgra's recall, it was unusual for peanut butter to be implicated in a disease outbreak. But the ConAgra case and the subsequent Peanut Corporation of America recall that was linked to nine deaths demonstrated that the pantry staple could harbour bacteria.
ConAgra's Paulsen said the company is committed to food safety and hasn't had any problems with its peanut butter since the recall.
"We took full responsibility in 2007, taking immediate steps to determine the potential causes of and solutions for the problem and acting quickly and definitively to inform and protect consumers," Paulsen said.
ConAgra officials blamed moisture in the production plant for helping salmonella bacteria on the raw peanuts grow. The company said the roof leaked during a storm and the sprinkler system malfunctioned, which allowed the moisture in.
The production plant was upgraded and ConAgra adopted new testing procedures before reintroducing Peter Pan peanut butter a few months later.
ConAgra said in documents filed with the Securities and Exchange Commission that it had already recorded charges of $31.7 million related to the investigation over the past nearly four years. The maker of Healthy Choice, Orville Redenbacher, Chef Boyardee and dozens of other branded foods said it doesn't expect this investigation to have a material effect on its profits.