Authorities in the northern German city of Hamburg said fresh tests indicated that cucumbers imported from Spain, initially suspected of making hundreds ill, may not be to blame.
Tests on two cucumbers revealed they carried the dangerous enterohamorrhagic E. coli (EHEC) bacteria, but not the strain responsible for the current massive contamination which has killed 15 in Germany and one in Sweden.
"As before the source remains unidentified," Hamburg's chief health official Cornelia Pruefer-Storcks told a news conference.
"Out of four cucumbers on which we have been able to confirm the presence of the EHEC pathogen, we have been able to establish -- as far as two of them are concerned -- that they carry the EHEC pathogen, but not the strain responsible for the current difficult developments," she said.
The two other cucumbers were still undergoing analysis, she added.
"Independently of the result of the two remaining tests, it was right to make public the results of our investigation as the contamination could very well cause EHEC," she said.
"It would have been irresponsible with such a number of ill people to keep quiet about a well-grounded suspicion.
"Protecting people's lives is more important than economic interests," she added.
Spanish fruit and vegetable sales have halted across nearly all Europe because of the outbreak, the industry's export federation said.
And both Spain and The Netherlands demanded compensation from the European Union because of the sudden slump in vegetable exports.
The situation is "extremely serious" for the farming sector, said Spanish Agricultural Minister Rosa Aguilar, estimating the loss to vegetable sales in Spain at more than 200 million euros ($288 million) a week.
"We are disappointed by the way Germany is handling the situation," she said, blaming officials who "pointed at Spanish cucumbers and Spain as the origin of this infection without having reliable data".
Germany remains convinced that raw vegetables are responsible for the virulent EHEC outbreak, and said it would offer subsidised loans to affected local farmers.
EHEC poisoning, in the worst of cases, can lead to full-blown haemolytic uraemic syndrome (HUS), a condition associated with bloody diarrhoea and kidney failure.
The Robert Koch Institute, Germany's national disease institute, said Tuesday it has recorded 373 confirmed cases of HUS, along with six deaths.
But regional authorities, who have been faster in reporting fatalities, said at least 15 people have died in Germany so far, mostly in the north, and more than 1,200 have been infected.
And in Sweden, the Soedra Aelvborg hospital in Boraas said a woman in her 50s who was treated for EHEC after a trip to Germany had died, in the first reported fatality outside the country.
The latest reported death in Germany was that of an 87-year-old woman who died in Paderborn, in North Rhine-Westphalia.
Late Tuesday, the Czech health ministry reported that a US tourist, a woman who was recently in north Germany, has been hospitalised in Prague with symptoms of E. coli infection.
Around Europe, other cases -- confirmed or suspected -- have been reported in Denmark, Britain, The Netherlands, Austria, France, Spain and Switzerland, all of them apparently stemming from Germany.
European health and consumer policy commissioner John Dalli on Tuesday said determining the source of the outbreak was "an absolute priority" and that the European Commission was working with the German authorities.
He added in his statement that it was "positive that the number of new infection cases seema to be declining", but stressed that vigilance was needed at this stage "as we are still working to pin down the possible source of the contamination."
Meanwhile, the Hygiene Institute at Muenster's University Clinic in western Germany announced it had put together a test to quickly identify people infected with the so-called Shiga toxin-producing E. coli.
The test allows identification within hours of the pathogenic agent in EHEC, the clinic said in a statement.
The agent is "especially virulent and able to resist antibiotics," the hospital said.
"This strand can be described as a hybrid or a chimera that combines different virulent traits," said Helge Karch, a professor at the Muenster clinic.