The Roma tomatoes tainted with salmonella that sickened at least 416 people in five states have been traced to farms in Florida and possibly South Carolina. The outbreak linked to Sheetz convenience stores infected people in 31 counties in Pennsylvania, Ohio, West Virginia, Maryland and Virginia with salmonellosis.

Food and Drug Administration investigators are visiting farms in one state this week. Florida officials were told that growers in five states that supplied tomatoes that could have ended up at Sheetz are being checked out.

Investigators likely will spend two weeks on the farms, but it could be "several weeks to months" before the agency identifies the cause of the outbreak or farms.

Past investigations of salmonellosis outbreaks have traced the source to humans, animals and contaminated water.