Was it a restaurant in Luebeck? A port festival in Hamburg? Or a yet-unknown site, still churning out deadly E. coli infections?

Speculation ran high Saturday about exactly where people were infected, especially since German health officials had no concrete proof yet about the source of the bacterial outbreak that has killed 18 people and sickened hundreds.

Local media reported Saturday that 17 persons fell ill with E. coli after they ate in the northern German city of Luebeck last month, and that researchers from Germany's national disease control center were inspecting the restaurant.

Other media said health experts were investigating whether the disease first spread at a festival in the northern city of Hamburg last month that was visited by 1.5 million people.

Those sickened at the Luebeck restaurant included eight women who participated in a workshop in the city, a group of tourists from Denmark and a child from southern Germany.

"The restaurant is not responsible for this, however by looking at their delivery companies, we may be able to find out how and from where the bacteria spread," Werner Solback, a microbiologist from the university hospital in Luebeck, was quoted as telling the Luebecker Nachrichten paper.

The situation in Germany is the deadliest E.coli outbreak in modern history. More than 1,700 people in Germany have been sickened, including 520 suffering from a life-threatening complication that can cause kidney failure. Ten other European nations and the U.S. have reported 90 other cases, all but two related to visits in northern Germany.

The news magazine Focus reported Saturday that the first patients were hospitalized a week after the Hamburg festival and said the national disease control center, the Robert Koch Institute, assumes that many people likely were infected there. The institute has not discussed this theory and could not be reached for comment Saturday.

German Health Minister Daniel Bahr, meanwhile, warned Saturday that the cause of the infection may still be active.

"We have to continue to be alert," Bahr told daily paper Ruhr Nachrichten. "Health officials are working nonstop to find the cause for all the infections, however, from outbreaks in the past we know that often the cause cannot be clearly identified."

Bahr reiterated the general warning to avoid eating cucumbers, tomatoes and lettuce.

Researcher Dag Harmsen at the Muenster University Hospital, which has been closely involved in the investigation, said that scientists were hoping to know enough about the E. coli strain by next week to be able to prevent new infections and better treat patients.

To avoid illness, the World Health Organization recommends that people wash their hands, keep raw meat separate from other food, thoroughly cook any food, and wash fruits and vegetables. Experts also recommend peeling raw fruits and vegetables if possible.