Investigators searching for the source of a killer bacteria were attempting to track supplies made to a north German restaurant, the German media reported on Sunday, as the death toll climbed to 22. The latest figures from the European Centre for Disease Prevention and Control (ECDC) recorded a further three victims to the previously confirmed 19. All but one of the deaths occurred in Germany, the source of the enterohaemorrhagic E. coli (EHEC) outbreak which has affected 12 countries. The other victim died in Sweden. Joachim Berger, the owner of the \'Kartoffelkeller\' restaurant in the Baltic seaport of Luebeck, told ZDF public television that health inspectors had carried out tests at the premises after three separate groups fell ill after eating there. Health officials, cited by Berger, said a family, a group of Danish tourists, and some women tax-inspectors were among the 17 who ate at his restaurant on May 13 and who contracted EHEC. Guests who became ill appeared to have eaten steak and salad. The Bild am Sonntag newspaper said a 48-year-old woman tax-inspector later died, while eight remain ill, two of them critically. Inspectors had thoroughly examined his kitchens and taken stool samples from all his employees, Berger said. None of the staff, who ate the same food as the guests, had been ill, he added. Christian Seyfert, a spokesman for the consumer protection ministry in the region, told AFP that speculation about the restaurant\'s link to the outbreak was unfounded. I have nothing to hide, I\'m sure everything is in order\" with the restaurant, the 67-year-old Berger told Bild am Sonntag. Berger said his food supplies came from a wholesaler in Moelln who obtained them from the central food market in Hamburg, the northern port where numerous cases of EHEC have also been reported. German Health Minister Daniel Bahr, who was on Sunday visiting Hamburg\'s Eppendorf University clinic where many of the region\'s EHEC patients are being treated, has warned that the source of infection could still be active. Related article: Women \'more at risk from E. coli outbreak\' \"Food health officials are working around the clock to identify the source of the infection,\" Bahr told the Ruhr Nachrichten newspaper on Saturday. \"But from earlier outbreaks we know that we can\'t always identify the source. \"It can\'t be ruled out that the source of infection is still active,\" he added, pointing to the need for continued vigilance as authorities still counsel against eating raw tomatoes, lettuce and cucumbers. Speaking to Bild am Sonntag, Bahr said also the situation in a number of north German hospitals, especially Hamburg and Bremen, was \"difficult\" because of the high number of admissions, adding that other hospitals would be called upon to help. Cases of E. coli poisoning have also been reported in more than 12 other countries, including Austria, Britain, the Czech Republic, Denmark, France, The Netherlands, Norway, Spain, Sweden, Switzerland and the United States. Each was related to German travel. Meanwhile, several German scientists Sunday suggested the outbreak could be linked to bacteria found in biogas plants. Biogas, or methane, is produced by the anaerobic digestion or fermentation of biodegradable materials such as manure, sewage and green waste. \"There are all sorts of bacteria which didn\'t exist before which are now produced in biogas fermentation tanks,\" Bernt Schottdorf, a medical analyst, told Welt am Sonntag newspaper. \"They crossbreed and mix with one another -- what goes on precisely hasn\'t really been studied,\" he said, adding that 80 percent of the production waste finds its way back onto fields as fertiliser. Ernst Guenther Hellwig, head of the veterinary and agriculture academy in Horstmar-Leer, said that because it had rained very little in the spring it was possible such fertilisers had not been washed off growing plants. \"Dangerous bacteria could be brought onto the fields this way and could contaminate vegetables,\" he said. The WHO has identified the bacteria as a rare E. coli strain never before connected to an outbreak of food poisoning. It is said to be extremely aggressive and resistant to antibiotics. The ECDC reported 1,605 cases of EHEC infection and 658 cases of the associated condition haemolytic uraemic syndrome (HUS) on Sunday.