The source of a killer bacteria remained elusive Monday after Germany announced that initial tests on suspected organic sprouts had proved negative, while the outbreak's death toll grew to 23.
The probes were carried out on a farm in the northern state of Lower Saxony after regional agriculture minister Gerd Lindermann said a link had been found to the main areas hit by the E. coli outbreak.
Results available from 23 of the 40 samples of seeds, water, ventilation and work surfaces tested indicated they were free of the bacteria responsible for 23 deaths and more than 2,000 people falling ill, the state's agriculture ministry said.
"Investigations are continuing," the ministry said, adding that it did not expect "any short-term conclusions."
Federal Agriculture Minister Ilse Aigner said Germany would maintain warnings against eating sprouts as well as tomatoes, lettuce and cucumbers, particularly in the north of the country, until the origin had been pinpointed.
But Andreas Hensel, head of the Federal Institute for Risk Assessment, said that "it is possible we shall never be able to identify the source" of the contamination.
Aigner said investigations were continuing with checks on vegetable supply chains and producers up and down the country.
She was to travel to Luxembourg on Tuesday to attend an emergency meeting of European Union agriculture ministers which is to discuss aid for farmers who are unable to sell their vegetables because of growing consumer fears.
At the meeting, EU health ministers will review the bloc's food safety alert system to ensure that warnings have "scientific basis and proof" before becoming public, according to EU health commissioner John Dalli.
Dalli said the alert system review was requested by Spain, where farmers were hit hard by inaccurate warnings that the outbreak might be linked to Spanish organic cucumbers.
Klaus Verbeck, who runs the farm in Bienenbuettel, some 80 kilometres (50 miles) south of Hamburg, told the Neue Osnabruecker Zeitung he uses no fertilisers for growing a variety of sprouts and had no idea how they might have been contaminated.
His farm, which produces sprouts for lettuce, azuki beans, mung beans, fenugreek, alfalfa and lentils and receives seed deliveries from several countries, has been ordered closed and all products recalled, authorities said.
"Salad sprouts are grown from seeds and water. They aren't fertilised. And there aren't any fertilisers used elsewhere on the farm," Verbeck said, alluding to the fact the E. coli bacteria may originally have come from animal droppings.
Similar outbreaks in Japan between 1996 and 2003 infected more than 10,000 people and left 22 dead, according to the Japanese health ministry.
The present outbreak, which has hit at least 14 countries, including the United States, has left at least 23 people dead, according to figures compiled by the European Centre for Disease Prevention and Control and German regional authorities.
Twenty-two died in Germany and one in Sweden -- a woman who had visited Germany.
In Germany, 1,601 patients have been diagnosed with EHEC and a further 630 with haemolytic uraemic syndrome (HUS), a life-threatening condition involving kidney malfunction, the Robert Koch Institute said.
The EHEC rate of infection grew from no more than nine a day during the first 10 days in May, to finally reach a peak of 122 on May 23 and has since slowed, the institute said.
The opposition Green party strongly criticised Chancellor Angela Merkel's government for its handling of the crisis.
"There is no crisis management. I really ask myself what the health minister and the consumer affairs minister are doing," said senior party official Renate Kuenast.
She went on to call for "national control measures" in a country where responsibility for public health matters is split between the federal government and authorities in each of the 16 states.