BERLIN - The foodborne bacterial outbreak that has hit Germany and other European nations is unlike anything Western experts have seen: 16 dead and more than 1,000 sick, including nearly 400 suffering severe and potentially fatal symptoms. But several days into the health threat, scientists remain unsure what produce - and what country - is responsible.

Investigators across Europe were frantically trying to determine the scope of the contamination by an unusual strain of the common E. coli germ - and where in the long journey from farm to grocery store the contamination occurred. German authorities pointed to a few cucumbers from Spain, but further tests showed that those vegetables, while contaminated, did not cause the outbreak.

In Germany, where the vast majority of deaths and severe illnesses have been reported, officials said that investigations including interviews with patients have shown that people were likely infected by eating raw cucumbers, tomatoes or lettuce. They are warning consumers to avoid those vegetables, and Russia went so far as to ban imports of those vegetables from Spain or Germany.

In its most severe form, the infection can attack the kidneys, sometimes causing seizures, strokes and comas.

It's "extraordinary" to see so many cases of the kidney complication from a foodborne illness, said Dr. Robert Tauxe, a foodborne disease expert at the U.S. Centers for Disease Control and Prevention.

"There has not been such an outbreak before that we know of in the history of public health," Tauxe said.

He added that the strain of E. coli in the European outbreak has not been seen previously in the United States. The CDC said two cases have been reported in the U.S., and that both people had recently traveled to Hamburg, Germany, where many of the infections occurred. The agency did not say what states the two people are in, but said it was working with state health departments to learn more about the cases and identify others.

There have been several high-profile foodborne outbreaks in the U.S. in recent years, but none with such a high death toll. There's little precedent in Europe, either. In 1996, an E. coli outbreak in the United Kingdom caused 216 cases and 11 deaths.

The CDC said two cases from the latest outbreak have been reported in the U.S., and that both people had recently traveled to Hamburg, Germany, where many of the infections occurred. The agency did not say what states the two people are in, but said it was working with state health departments to learn more about the cases and identify others.

The World Health Organization said 86 percent of those sickened were adults, and two-thirds were women. It said it was unusual that more children weren't affected.

E. coli is found in the digestive systems of humans, cows and other mammals. It has been responsible for a large number of food poisoning outbreaks around the world. In most cases, it simply causes diarrhea and other non-lethal stomach ailments.

But the bacteria involved in the latest outbreak, EHEC, causes more severe symptoms, ranging from bloody diarrhea to hemolytic uremic syndrome - the rare kidney condition that the most seriously ill patients are suffering from. At least 373 people in Germany and 15 in Sweden have come down with the syndrome, which normally kills roughly 5 percent of the patients who get it.

The syndrome is not just a short-term problem: Ten to 20 years after they recover, between 30 percent and half of survivors will have some kidney-caused problem, experts say.

A Swedish woman who died early Tuesday was the first, and so far the only, fatality linked to the outbreak outside Germany.

The latest German death occurred Tuesday in the northwestern city of Paderborn, but German regional officials said they are seeing a sharp drop in the number of new cases.

Total cases, including less severe EHEC infections, top 1,150 in Germany alone. Other infections have been reported in Denmark, France, the Czech Republic, the U.K., the Netherlands and Switzerland. Many patients in those cases had recently traveled to Germany.

The highly politicized mystery deepened with new evidence that German vegetables may have been contaminated by at least two strains of EHEC, or enterohaemorrhagic E. coli.

Authorities in Hamburg said last week that they had detected EHEC on four cucumbers in a market in the northern German city, three imported from Spain and the fourth of unclear origin.

European Union officials said Germany identified cucumbers from the Spanish regions of Almeria and Malaga as possible sources of contamination, and that a third suspect batch, originating either in the Netherlands or in Denmark and sold in Germany, was also under investigation.

On Tuesday, however, officials said they had found a slightly different type of EHEC on the cucumbers than the strain detected in the feces of sick people in Germany. That means those cucumbers did not cause the outbreak, but posed a health risk nonetheless, the German officials said.

Spain's agriculture minister, Rosa Aguilar, seized on the find as evidence that "our cucumbers are not responsible for the situation." Spain exports most of its produce to other countries in Europe.

Denmark said no traces of EHEC bacteria were found in tests of vegetables conducted there over the weekend. Exports of Dutch cucumbers to Germany were halted but authorities said tests of a cucumber grower and a warehouse found no EHEC bacteria there, either.

The northern state of Schleswig-Holstein, which neighbors Hamburg and is one of the worst affected, said tests on 141 samples of food including cucumbers, tomatoes, milk products, zucchini and poultry had found no EHEC. Mecklenburg-Western Pomerania, another northern state, has been unable to find EHEC in 38 samples so far.

Russia's chief sanitary agency on Monday banned cucumbers, tomatoes and fresh salad from Spain and Germany, and said it may extend the ban to all European Union member nations because of questions about the source of the infection.

Cheng reported from London. Mike Stobbe in Atlanta, Geir Moulson in Berlin and Karl Ritter and Malin Rising in Stockholm contributed to this story.