Investigators still don't know what caused the summer salmonella outbreak among patrons of Sheetz convenience stores, which resulted in 429 confirmed cases among people in nine states.

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The search for the source of contamination had a setback in August, when Hurricane Charley hit Florida, home of farms that likely grew the tainted tomatoes at the heart of the outbreak.

But once investigators finally conducted their farm work, they determined that food safety practices, called for in the 1990s following tomato-related outbreaks, had, in fact, been adopted.

That good news about past practices, however, left officials struggling to understand the nature of the new problem.

"We don't know how the contamination moved to the produce," said Jack Guzewich, director of emergency coordination and response for the Center for Food Safety and Applied Nutrition at the Food and Drug Administration. "The other apparent thing is -- and this isn't proven -- the organism can survive in the environment, on the produce, in the face of the [safety] things that are being done.

"What does that mean? What are the best steps to take? Those are the questions we'd like to answer."

While the number of confirmed cases in the Sheetz outbreak doesn't make it one of the top 10 of its kind in the United States, its local impact was huge.

In Pennsylvania, 288 salmonella cases were conclusively linked to the outbreak, although officials believe many more here were actually sickened. Coming within a year of 660 people getting sick with hepatitis A after eating green onions at the Beaver Valley Mall Chi-Chi's, the outbreak at Sheetz was yet another reason for consumers here to think twice before eating produce from a food vendor.

Pittsburgh-area residents have a keen appreciation for an emerging trend in food safety that might not be recognized in other communities, said Dr. Andre Weltman, a public health physician with the state Department of Health. Whereas food safety concerns about meat have been paramount in the public mind, he said, more and more outbreaks involve produce.

Neither the Sheetz nor the Chi-Chi's outbreak was the result of poor food handling practices at either establishment. But there was an important difference between the two: Green onions served at Chi-Chi's were grown in Mexico while the tomatoes sold at Sheetz were grown in the United States.

"I think there's sometimes a tendency to blame these things on places that are far away," said Weltman. "So, it's a bigger problem and will take more systemic solutions than just saying, 'Oh, it's one dirty farm or one dirty country.'"

Investigators believe that as many as five different strains of salmonella bacteria contaminated the tomatoes served at Sheetz, said Dr. Amy DuBois, epidemic intelligence service officer with the Centers for Disease Control and Prevention. The bacterial variety virtually eliminates the chance that contamination came from an infected food-handler somewhere along the food distribution chain, DuBois said.

The most common reservoirs for the roughly 2,000 known strains of salmonella are wild and domestic animals, so investigators tend to think the contamination occurred either at the farm or in packing sheds where tomatoes are processed. This is surely cold consolation for the owners of Coronet Foods, the Wheeling, W.Va., company that closed this year after it was revealed that Coronet sliced the tomatoes served at Sheetz.

"Our basic assumption is that the initial contamination happens probably at the growing or packing shed stage of the operation, and it is exacerbated downstream from there, but we can't prove that," said Guzewich, the FDA official.

Investigators hoped they would be able to recommend safety steps like those called for in the 1990s, following two salmonella outbreaks involving tomatoes. Nearly 300 people in Midwestern states were sickened during those outbreaks, both of which involved tomatoes packed in a single facility in South Carolina.

In that case, the packer received tomatoes from 12 growers operating within several miles of the facility. Tomatoes were dumped into a heated, chlorinated water bath in a roofed receiving area outside the packing house.

Chlorine is key to preventing contamination from bird droppings and other organic debris that can find their way into a tank, but chlorine levels were not being routinely monitored or recorded, investigators found. They also determined that tomatoes placed in water cooler than the tomato pulp will absorb water and salmonella organisms into the core tissues through the stem scar. Salmonella can be transferred from the stem scar into the tomato by a knife blade used to cut a tomato.

When investigators visited farms that grew the Sheetz tomatoes, they were pleased to see that chlorine levels were, in fact, being monitored, said DuBois of the CDC. What's more, water temperatures were being kept up.

Investigators focused on one farm in Florida, DuBois said, and the FDA might visit that operation again this spring to observe conditions during the planting of a tomato crop.

The salmonella story isn't over for the Sheetz chain, either.

With 307 stores in six states, Sheetz derives revenue from more services than just made-to-order sandwiches, said Steve Sheetz, the company chairman. So, the overall financial results for the year were "very good," he said. But the sandwich business was hurt, and it hasn't completely bounced back, particularly in areas close to Pittsburgh.

Sheetz sells everything from gasoline to cigarettes these days, but it started out as a dairy store and restaurant. So, selling food and dealing with food safety issues has been at the core of the business since the 1950s, Sheetz said.

Even so, the company had never before suffered a problem like the salmonella outbreak. Many feel the company's response contrasted favorably with that of Chi-Chi's, where executives weren't nearly so quick to respond publicly to the outbreak and communicate openly with customers. Being upfront with people, whether the news is good or bad, is just the way the company operates, Sheetz said.

While the company survived the outbreak, Sheetz said the fate met by Coronet is chilling.

"I think that speaks to how fragile success is -- how fragile that line is between success and failure," he said.