ConAgra Foods said Thursday that moisture from a leaky roof and faulty sprinkler was the source of the salmonella bacteria that contaminated peanut butter at its Georgia plant last year, sickening more than 400 people nationwide.

The Omaha-based company conducted a nearly two-month investigation into the contamination and pledged to ensure that Peter Pan peanut butter is safe when it returns to stores in mid-July.

"Consumer safety and health is our top priority," ConAgra spokeswoman Stephanie Childs said. "We plan to do our best to regain consumer trust once Peter Pan returns to stores."

Childs said the company traced the salmonella outbreak to three problems at its Sylvester, Ga., plant last August.

The plant's roof leaked during a rainstorm, and the sprinkler system went off twice because of a faulty sprinkler, which was repaired.

The moisture from those three events mixed with dormant salmonella bacteria in the plant that Childs said likely came from raw peanuts and peanut dust.

She said the plant was cleaned thoroughly after the roof leak and sprinkler incidents, but somehow the salmonella remained and came in contact with peanut butter before it was packaged.

ConAgra recalled all its peanut butter in February after federal health officials linked it to cases of salmonella infection. At least 425 people in 44 states were sickened, and numerous lawsuits have been filed against the company.

The recall covered all Peter Pan peanut butter and all Great Value peanut butter made at the Sylvester plant since October 2004. That plant is ConAgra's only peanut butter plant.