Having been tripped up by several apparently false leads, German authorities had zeroed in on organic bean sprouts as the possible origin of the contamination with a highly virulent strain of E coli.
But initial probes carried out on a farm growing a variety of salad sprouts in the northern state of Lower Saxony proved negative.
Another lead in Hamburg, the epicentre of the outbreak, involved salad sprouts from the same farm found in the refrigerator of a sickened man. But that sample also failed to display any trace of the bacterial strain.
European Agriculture Commissioner Dacian Ciolos urged Germany to quickly pinpoint the driver of the lethal bacterial strain, warning that consumers were losing confidence every day the outbreak remains a mystery.
“Without this answer, it will be difficult to regain the trust of consumers, which is essential for the market to regain its strength,” he said at emergency talks in Luxembourg.
Health officials in Hamburg had identified organic cucumbers from Spain as the culprit last month before backtracking when tests came up negative.
The bacterium has killed at least 22 people, 21 in Germany, after two more deaths were reported yesterday. The other fatality was a woman in Sweden who had recently returned from Germany.
The outbreak has left more than 2,300 ill, with symptoms ranging from bloody diarrhoea to kidney failure.
The Robert Koch Institute, Germany’s national health centre, said the rate of reported infections appeared to be tapering off.
Meanwhile, German officials defended their decision to issue warnings on vegetables to the public even before full test results were known.
But opposition members have criticised the slow pace of the probe, suggesting that the country’s federal system was partly to blame because public health matters are dealt with by both national and state authorities.
Lower Saxony agriculture minister Gert Lindemann, who had said there were “clear indications” of a connection between a local salad sprout farm and areas contaminated, admitted a smoking gun might never be found.
“It’s quite possible that we’ll never find the active contaminant,” Mr Lindemann told the Bild newspaper.
“But this doesn’t take away from statements warning about salad sprouts.
“The warning goes for as long as investigations are under way and suspicions have not been laid to rest,” he said.
The head of the Federal Institute for Risk Assessment, Andreas Hensel, has also warned that “it is possible we shall never be able to identify the source” of the contamination.
Similar outbreaks in Japan between 1996 and 2003 infected more than 10,000 people and left 22 dead, according to the Japanese health ministry.
Radish sprouts were widely suspected, but scientists were never able to prove the connection and farmers successfully sued for compensation.