The salmonellosis outbreak that sickened at least 416 people in five states might have been caused by four bacterial strains, an unusual occurrence, health officials said Friday.

Investigators suspect that all strains were found on contaminated Roma tomatoes served at convenience stores owned by Altoona-based Sheetz. Officials said they believe the tainted tomatoes have been removed from the market and are no longer infecting people.

Finding multiple strains of salmonella in a single outbreak is unusual, but it has happened several times, said Jennifer Morcone, CDC spokeswoman. Three strains of salmonella were found in beef jerky that sickened 93 people in New Mexico in 1995, and two salmonella strains were associated with orange juice that sickened people in Florida.

An animal could carry more than one strain and contaminate food growing in the field, the CDC said.