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“Public health investigations are not always successful. But a big one with a lot of investigation around it is usually successful,” said Dr. Robert Tauxe, a foodborne disease expert at the U.S. Centers for Disease Control and Prevention.

To nail down the source, scientists will have to match the strain found in patients to one in vegetables or other sources by using DNA sequencing, said Brendan Wren, professor of pathogen molecular biology at the London School of Hygiene and Tropical Medicine.

But it can be difficult to find the strain in vegetables, and Wren doubts cucumbers are responsible. “As in many foodborne disease outbreaks, the culprit may never be identified and the epidemic just fades away,” he said.

Meanwhile, investigators will increase efforts to find the food distributors and producers where the vegetables originated.

That can take weeks or even months, and can be complicated by the fact that different vegetables are often eaten together, as in salads, Tauxe said.

In the U.S., the government said it would step up testing of any imports of cucumbers or other possibly implicated produce from affected countries _ but the nation gets very little fresh produce from Europe, especially this time of year. There was just one shipment of cucumbers from Spain in May, for instance, and no cucumbers, tomatoes or lettuce from Germany since January.

Another challenge for health officials: Catching and preventing future outbreaks of this strain.

According to an expert panel of the European Food Safety Authority, there is limited data on the presence of dangerous E. coli strains across Europe.

Current surveillance systems aren’t well coordinated across Europe, the group said. It recommends monitoring a number of dangerous E. coli strains _ but not the one that is responsible for the current outbreak.

In Germany, there are no spot checks of imported food coming from the 25 countries that are part of a zone that lacks internal border controls.

In the United States, labs regularly test for a dangerous E. coli type in stool samples from people with food-poisoning symptoms, but only a small percentage of the labs test for other forms of E. coli that make people sick. In recent years, investigators have found that a wider variety of E. coli bugs are also causing illness.

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Stobbe reported from Atlanta. Medical Writer Maria Cheng in London and Karl Ritter in Stockholm contributed to this report.