A top World Heath Organization official says the source of Germany's deadly E. coli outbreak may never be known.

Dr. Guenael Rodier, WHO's director of communicable diseases, tells the Associated Press that investigators must find the culprit within a week. He says after that, it would become difficult to link patients with what they ate. Rodier says the contaminated vegetables have probably already disappeared from the market.

The E. coli outbreak has killed at least 23 people and sickened more than 2,400 since late May. Nearly all the victims live in or visited northern Germany.

The source of the virus is a mystery. Investigators have already ruled out organic sprouts, lettuce, tomatoes and cucumbers from Spain.

European Union Health Commissioner John Dalli is cautioning authorities not to rush to judgement on the source of such infections until there is more proof. He says such rapidly reported news spreads fear and causes problems for farmers.

EU Farm Commissioner Dacian Ciolos has recommended a $220-million aid package to help farmers recoup some of their losses from unsold vegetables because of the E. coli crisis.

Russia and some other countries have banned all vegetables from the EU. Spanish Agriculture Minister Rosa Aguilar is demanding full compensation from Germany for losses suffered by farmers in her country.

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