The agency is still unsure of the origin of the contaminated fruit, though it has focused on central Florida and Mexico.

Federal officials said Thursday that they might never learn which farms produced tainted tomatoes that have now sickened 228 people in 23 states with a rare form of salmonella.

"At this stage of the investigation there is no guarantee that we will be able to trace the outbreak back to the farm level, although that is the goal," David Acheson, the Food and Drug Administration's associate commissioner for foods, told reporters Thursday.

The agency is still unsure of the geographic area that is the source of the contaminated fruit, though it has focused its probe on growing regions in central Florida and Mexico.

Discovering the source of the salmonella would enable health officials to give the public an all-clear on eating tomatoes and would relieve the financial pressure on farmers, who have been destroying the fruit and losing millions of dollars because shoppers couldn't figure out what tomatoes were safe to consume.

It also would provide crucial information that could prevent future outbreaks.

"We want to get on that farm as quickly as possible and do some environmental sampling and figure out what went wrong," Acheson said.

He said the investigation was proceeding slowly because the tomatoes weren't sold with bar codes and were aggregated into big lots from multiple farms and suppliers.

The slow pace of the investigation has prompted calls for better enforcement on the part of the FDA.

On Thursday, Consumers Union said that the federal agency should increase inspections of food processing plants and that Congress should give the FDA broad mandatory recall authority in light of the recent outbreaks of illness linked to produce.

"The FDA has been understaffed and underfunded for far too long. At the very least, the agency's budget for inspections must be increased so that it is visiting produce processing plants annually, not just once every five to 10 years," said Jean Halloran, director of food policy initiatives at Consumers Union.

Legislators also heaped criticism on the FDA for the slow pace in the development of a formal food protection plan, a food safety initiative announced by the Bush administration in November.

"Today is no different. We face yet another food crisis. . . . It has sickened people, devastated an entire industry and cost consumers, producers and retailers millions of dollars. Tragically, similar to food crises in the past, FDA has been unable to identify the source of this contamination or even where the tomatoes originated," said Rep. John D. Dingell (D-Mich.).

This is the 13th outbreak of salmonella linked to tomatoes since 1990.

In an interview with The Times, FDA Commissioner Andrew C. von Eschenbach conceded that the agency has room for improvement when it comes to food safety.

"I don't think we have done the job yet. We may be facing it for the 13th time and we may have begun to address it earlier, but I don't think we have reached a point where we have put all the parts and pieces in place that are necessary to bring this closer to zero defects," he said.

Meanwhile, the size of the outbreak continues to grow.

"We would characterize this outbreak as ongoing," said Ian Williams, a medical epidemiologist at the Centers for Disease Control and Prevention.

The number of confirmed cases has jumped 37% since Monday as health departments in more states have become aware of the problem. Williams said health officials have learned of people becoming ill as recently as June 1. The first illness was reported in April.

The FDA has told consumers they can safely eat cherry and grape tomatoes and tomatoes still attached to the vine when sold in stores. People should avoid plum, Roma and standard round tomatoes.

California tomatoes of any variety have been declared safe for consumption because the investigation has ruled out the state as a potential source.

"If you don't know where the tomato has come from, don't take the risk. The obvious advice is don't consume the tomatoes," Acheson said. He added that retailers should determine the source of their tomatoes and other produce as a prudent business practice.