At least 16 people, including one in Sweden, have died from a virulent form of enterohamorrhagic E. coli (EHEC), which can cause bloody diarrhoea and kidney failure known as haemolytic uraemic syndrome (HUS).
But contrary to early indications, imported cucumbers from Spain do not appear to be behind the outbreak. Two samples from a Hamburg market contained EHEC bateria but not the deadly O104 strain.
"Our hope to discover the source of serious HUS complications unfortunately was not realized," said Hamburg's Health Minister Cornelia Prüfer-Storcks on Tuesday. "As before, the source remains unidentified."
Prüfer-Storcks, however, defended last week's decision to link the outbreak to Spanish produce.
"Regardless 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 this number of ill people to keep quiet about a well-grounded suspicion. Protecting people's lives is more important than economic interests."
Several European countries have banned Spanish vegetables, sparking criticism from Madrid amid huge losses for Spain's farmers.
Spanish Agriculture Minister Rosa Aguilar denied before the results were released in Hamburg that her country was the source of contamination.
"From the beginning, in Germany, Spanish cucumbers have been named as responsible for this situation. We must say that it is not true and we must demand that the Germany authorities wrap up their investigation immediately," she said.
Aguilar called for a "European solution" and slammed Germany's handling of the outbreak.
The situation is "extremely serious" for the agriculture sector, Aguilar said, estimating Spain's vegetable sale losses at "more than €200 million ($288 million) a week."
The Spanish fruit and vegetable producer-exporter federation said sales have halted across nearly all of Europe as the scare rippled across the continent.
Asked which countries had stopped buying Spanish produce, federation president Jorge Brotons told a news conference: "Almost all Europe. There is a domino effect on all vegetables and fruits."
Germany's Health Minister Daniel Bahr warned earlier on Tuesday that the number of cases is likely to grow. "The infection source remains active and we have to reckon with a growing number of cases."
The Robert Koch Institute (RKI), 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 Södra Älvsborgs hospital in Boraas said a woman in her fifties 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 the western state of North Rhine-Westphalia.
The Hygiene Institute at Münster's University Clinic in western Germany meanwhile 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," according to Professor Helge Karch from the Münster clinic.
The Stockholm-based European Centre for Disease Prevention and Control has described the outbreak as "one of the largest worldwide and the largest ever reported in Germany."
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.