BERLIN—Authorities say a single batch of deep-frozen strawberries appears to have been behind an outbreak of gastroenteritis in eastern Germany that hit more than 11,000 people, mostly children at schools and day-care centers.

The national disease-control center, the Robert Koch Institute, said late Friday that studies showed a strong connection between eating food prepared with the strawberries and the cases of vomiting and diarrhea.

It says evidence has increased that norovirus, a mostly food- or water-borne illness, was the cause—though it can't yet entirely rule out other pathogens.

Agriculture Ministry spokesman Holger Eichele told news agency dapd that the strawberries are believed to have come from outside the European Union.

Officials have said the sicknesses were moderate and most children recovered quickly without needing to be hospitalized