Washington —

Despite months of effort, federal and state investigators may never learn the source of the tomatoes that caused a salmonella outbreak that may have sickened thousands, officials said Thursday.

For two months, the Food and Drug Administration, along with the federal Centers for Disease Control and Prevention (CDC) and state public health officials, have been investigating the multi-state outbreak of salmonella infection, which can cause diarrhea, fever and abdominal cramps 12 to 72 hours after eating tainted food.

The epidemiological investigation has narrowed the problem to raw red plum, red Roma or red round tomatoes. And the evidence suggests the tainted fruit came from Florida or Mexico, where farmers were harvesting when the earliest known victim fell ill on April 10.

Florida "fits with the time frame," and investigators have not found evidence that could rule out portions of the state, David Acheson, the Food and Drug Administration's associate commissioner for foods, said Thursday in a conference call with reporters. Although no illness has been reported in Mexico, that country has not been ruled out as a possible source, he said.

Acheson said investigators may never know exactly which farm or packing house may have caused the problem because so many factors are involved. "The trace-back continues, but we still have not identified a specific source of the outbreak," he said.

Investigators may track a problem to a single packer, and then learn that each box was packed with tomatoes selected from many different growers to fill an order requesting, say, "small, ripe" tomatoes, he said.

At that point, investigators can't identify the source of any one tomato in the box.

Investigators have not yet been dispatched to individual farms because "we don't know yet where to send them," Acheson said.

"Tomatoes are notoriously difficult to trace," he said.

Acheson said he could not say exactly how many public health officials are are involved in the combined state and federal efforts, but similar investigations in the past have brought together several hundred epidemiologists and other experts.

Ian Williams, chief of the CDC's OutbreakNet Team, said on the conference call that officials don't know if more people will get sick because the raw tomatoes could still be in salsas, guacamole or other raw foods.

"We're still characterizing this as ongoing," with the most recent case being confirmed on June 5, he said.

Since April, investigators have confirmed 383 victims in 30 states and the District of Columbia. All had been infected by the same strain of salmonella, as identified by its genetic fingerprint.

Williams said that most likely, this particular salmonella outbreak has sickened thousands of people, but most cases have not been confirmed by tests.

So far, the outbreak has not been blamed for any deaths, but the CDC said a Texas man suffering from cancer was infected with salmonella at the time of his death and the illness may have contributed to his death.

The CDC says most people recover from infection without treatment, though some need antibiotic treatment. Infants, elderly people and those with impaired immune systems are more likely than others to get seriously ill.

In the most severe cases, salmonella could spread from the intestines to the bloodstream and cause death. Acheson said that given the potential for death, "I don't believe for one minute we have overreacted" by warning consumers about the bad tomatoes.

One year ago this month, the FDA launched a Tomato Safety Initiative in response to recurring salmonella outbreaks tied to fresh tomatoes. The agency said it would be sending officials to selected tomato farms and packing facilities to study environmental factors, such as "irrigation water, wells, procedures for mixing chemicals, drought and flooding events, and animal proximity to growing fields."

Acheson said the agency is continuing to work with industry to come up with better ways to handle future outbreaks.