Berlin (CNN) -- No trace of the E. coli bacteria has been found in initial tests at a German sprout farm suspected of being the source of the outbreak that has killed at least 22 people, agriculture officials in the state of Lower Saxony said Monday.

But authorities said that does not mean their suspicions are not correct; they would not expect to find evidence of E. coli if the tainted sprouts were no longer in the supply chain.

Test results are back for 20 of the 40 samples, Lower Saxony officials said Monday. It was not clear when the rest of the test results would be available.

On Sunday, officials said German-grown sprouts are the likely source for the E. coli outbreak.

"There is a direct link between this company and these people getting sick," said Gert Lindemann, agricultural minister in Lower Saxony.

It was not clear how the rare strain may have gotten into the sprouts, officials said. E. coli has not been found in the company.

The company, Gaertnerhof, has halted production and is recalling its products.

Gaertnerhof, in the town of Bienenbuettel, said in a statement it was "shocked and worried. ... that part of our production has been linked to E. coli infections" and had never had a problem in its 25 years of growing sprouts.

It said it had found no evidence of E. coli during routine testing in January or during tests last month in response to the health scare.

But a leading German microbiologist said Monday that sprouts were a "plausible" source of the infection.

Sprouts can harbor bacteria, which can spread during the growing process, said Alexander Kekule of the University of Halle-Wittenberg.

"Either it was inside the seed, which I do not think is the case, or the bacteria was inside the water," Kekule said on Germany's NDR radio. He is not working directly on the case.

Sprouts are bred in drums that are heated to slightly warmer than body temperature -- ideal conditions for the growth not only of sprouts, but of bacteria, the agriculture ministry of Lower Saxony said.

Authorities say the sprouts may have become tainted so long ago that trace-back tests may never identify the company as the source. But several restaurants and cafeterias linked to the outbreak got sprouts from the company, officials said.

And two workers at the agricultural company have come down with severe cases of diarrhea; in at least one of those cases, E. coli was the cause, Lindemann said.

Bienenbuettel is in the district of Uelzen in north-central Germany.

The outbreak of the virulent strain of E. coli has infected more than 2,200 people in at least 12 countries, European health authorities said Sunday.

All but one of the 22 fatalities were reported in Germany, where officials said it was not clear whether the outbreak had peaked. One person died in Sweden after visiting Germany.

Last week, Spain rejected suggestions that its cucumbers could have been the source after the European Food Safety Alert Network said E. coli bacteria were found in organic cucumbers originating from Spain, packaged in Germany and distributed to various countries.

Authorities initially called for the cucumbers to be pulled from sale.

Germany later said Spanish produce was not the source of the infection, and Spanish farmers are now demanding hundreds of millions of euros in compensation.

Jose Maria Pozancos, the head of the Spanish fresh produce exporters group Fepex, said he wants Germany to apologize, reimburse Spanish farmers at least 400 million euros ($584 million) for their losses, and help Spain repair damaged consumer confidence.

Spanish produce exporters have seen a 40% decline in demand since the crisis began, Pozancos said.

Germany has seen 630 cases of hemolytic uremic syndrome (HUS) -- a form of kidney failure -- in the current outbreak, according to the European Centre for Disease Prevention and Control. That's more cases of HUS than in any other recorded outbreak, worldwide.

Fifteen patients in Germany have died of HUS, according to the center, while six died of enterohemorrhagic E. coli, a strain that can result in abdominal cramps and bloody diarrhea and causes intestinal hemorrhaging.

The center said 1,601 people have that E. coli strain but do not have HUS.

Infections have also been identified in Austria, Czech Republic, Denmark, Finland, France, Netherlands, Norway, Poland, Spain, Switzerland and the United Kingdom, according to the organization.

Two women and a man who traveled last month to northern Germany were hospitalized in the United States with HUS as of Friday, Chris Braden of the U.S. Centers for Disease Control and Prevention said. A fourth person developed bloody diarrhea, but was not hospitalized, he added.

Two U.S. service members in Germany also developed diarrhea, Braden said. "We have no expectation that this will spread in our country," he added.

The U.S. government website foodsafety.gov says that since 1996, "there have been at least 30 reported outbreaks of food-borne illness associated with different types of raw and lightly cooked sprouts. Most of these outbreaks were caused by Salmonella and E. coli."

The World Health Organization says that in 1996, "an outbreak linked to contaminated radish sprouts in school lunches caused 9,451 cases" of E. coli in Japan.

CNN's Frederik Pleitgen, Al Goodman and Per Nyberg contributed to this report.