The cause of a deadly E. coli outbreak in Germany remained a mystery Monday after officials ruled out organic sprouts as the culprit.

Officials had said on Sunday that sprouts from a farm in Lower Saxony state were the source of the bacteria that has killed at least 22 people and sickened more than 2,200 others.

But they backtracked Monday after the the state's agriculture ministry said most of the samples taken from the farm had tested negative. Officials have also ruled out lettuce, tomatoes and cucumbers from Spain.

European Union agriculture ministers meet in Luxembourg Tuesday to talk about the crisis and its economic impact. Russia has banned all vegetables from the EU and the ministers will discuss compensating farmers.

This is the deadliest E. coli outbreak in modern history. Nearly all the victims are German. Eleven other European nations and the United States also report E. coli cases and say most of the victims had visited northern Germany.

E. coli symptoms include stomach cramps, diarrhea, fever and vomiting.