Germany's agriculture minister Ilse Aigner said experts are working round the clock to find the source of the unusual strain of the bacteria.
Vegetables are believed to have spread the outbreak but it is not known where, on the trip from field to shop, the contamination took place.
As well as those killed, more than 1,000 people have fallen ill, with 400 having severe, possibly deadly symptoms.
"Hundreds of tests have been done and the responsible agencies ... have determined that most of the patients who have been sickened ate cucumbers, tomatoes and leaf lettuce and primarily in northern Germany," Mr Aigner said. "The states that have conducted the tests must now follow back the delivery path to see how the cucumbers, or tomatoes or lettuce got here."
Contaminated cucumbers in Spain were initially blamed but additional tests showed they did not trigger the outbreak. However, Germans have been told to avoid eating the vegetable as well as tomatoes and lettuce.
E. coli outbreaks are fairly common in Britain and Europe but it is rare for them to lead to deaths.
However, the outbreak centred on Germany - enterohaemorrhagic E.coli, or EHEC - is more serious, ranging from bloody diarrhoea to the rare hemolytic uremic syndrome (HUS) in which the kidneys are attacked, leading to seizures, strokes and comas.
In Germany, at least 373 people have been affected by HUS.
According to the World Health Organisation, there have been 400 cases of HUS and 843 of EHEC across Europe.
Austria, Denmark, Germany, the Netherlands, Norway, Spain, Sweden, Switzerland and the UK have all seen cases. Apart from two cases, those affected live in Germany or had had recently visited northern Germany.