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He said the contamination could be at such a low level that tests wouldn’t pick it up and that to narrow down the source, more detailed studies of patients _ what they ate and where _ were necessary.

He also recommended that authorities should then trace back those food sources to their suppliers _ which is exactly what led German officials to single out the sprout producer as a possible source, linking it to several restaurants where more than 50 people fell ill.

Since 1996, about 30 outbreaks of foodborne illness in the U.S. have been linked to raw or lightly cooked sprouts. Sprouts were also implicated a 1996 E. coli outbreak in Japan that killed 12 people and reportedly sickened more than 9,000.

The Lower Saxony ministry statement left consumers across the continent still puzzled as to what is safe to eat and warned that it was not clear how soon an answer would be found.

“A conclusion of the investigations and a clarification of the contamination’s origin is not expected in the short term,” the ministry said.

Andreas Hensel, the head of Germany’s Federal Institute for Risk Assessment, added that it might never be possible to establish the outbreak’s cause.

“We have to be clear on this: Maybe we won’t be able anymore to identify the source,” he told reporters in Berlin.

At an European Union health ministers meeting Monday in Luxembourg, Germany defended itself against claims it had acted prematurely in pointing toward Spanish cucumbers.

“The virus is so aggressive that we had to check every track,” said Health State Secretary Annette Widmann-Mauz.

The EU will hold an emergency meeting of farm ministers Tuesday to address the crisis and its economic impact, including a ban by Russia on all EU vegetables.

At the organic farm in Bienenbuettel, between the northern cities of Hamburg and Hannover, there was no immediate reaction Monday by the owners. The gates remained locked. Two security guards patrolled while scores of television satellite trucks and journalists waited outside and a TV helicopter circled overhead.

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Kirsten Grieshaber in Berlin, David Rising in Bienenbuettel, Raf Casert in Brussels and Daniel Wools in Madrid and AP Medical Writer Maria Cheng in London contributed to this report.