Police said today a criminal investigation is under way in Sweden to determine how an intestinal parasite ended up in the town of Ostersund’s municipal water supply, sickening more than 2,000 residents.

Environmental prosecutor Christer B. Jarlas said release of the parasite cryptosporidium may have been the result of criminal negligence, the Swedish news agency TT reported Tuesday.

He said he has reason to believe that the contamination was due to carelessness by one or several individuals who didn’t have control of their operations.

Sweden’s National Center for Infectious Disease Control told TT the 50,000 residents of Ostersund will have to boil their water for several weeks.