Preliminary tests had shown the sprouts as a possible source, and authorities had shut down the farm and recalled all its produce.
The Lower-Saxony state agriculture ministry said 23 of 40 samples from the sprout farm suspected of being behind the outbreak have tested negative for the highly aggressive, "super-toxic" strain of E. coli bacteria. It said tests were still under way on the other 17 sprout samples.
"The search for the outbreak's cause is very difficult as several weeks have passed since its suspected start," the ministry said in a statement, cautioning that further testing of the sprouts and their seeds was necessary to achieve full certainty.
Negative test results on sprout batches now, however, do not mean that previous sprout batches weren't contaminated.
Officials think more E. coli cases could arise because contaminated sprouts may have already been delivered to restaurants and grocery stores.
The outbreak has killed 22 people and sickened more than 2,200 others.
E. coli is found in the feces of humans and livestock and can spread to produce through sloppy bathroom habits among farmworkers or animal waste in fields and in irrigation water.
The Associated Press contributed to this report.
