Östersund municipality believe the Cryptosporidium parasite entered the municipal water system originated in a multi-family dwelling in the city's Odensala neighbourhood.
“We've found high levels of the parasite in the connection to this source. I can't say much more. We've handed over the information to the police who are responsible for the investigation. A property owner will also be contacted by (municipal water administrator) Vatten Östersund,” said Östersund environmental head Jari Hiltula to the TT news agency.
The flow from the contamination source has since been shut off.
Police in Jämtland county say they suspect the building in Odensala is the source of the contamination. City administrator Bengt Marsh confirmed that the municipality has given information to the police that the source of the parasite is a single building.
According to the local Östersunds Posten (OP) newspaper, police suspect that a sewage line leading away from the building was mistakenly connected to a rainwater drainage system.
“It looks like the sewage pipe wasn't connected properly,” Andrew Sörensson, an environmental crimes investigator with the Östersund police, told the newspaper.
The worst of the wave of illnesses caused by the Cryptosporidium parasite appears to be over. Nevertheless, the municipality's website has received an additional 500 reports of people suffering from stomach problems.
Altogether, 11,200 people have reported coming down with symptoms likely caused by the parasite.
On Wednesday afternoon, ultra-violet equipment arrived in Östersund which will be placed in the city's water treatment plant to help kill off the parasite.
“We're going to work with the supplier to get it up and running as fast as we can. I don't want to speculate about how long it might take,” said Richard Jonsson, head of Vatten Östersund, to TT.
Both the police as well as city officials involved in the investigation say it remains unclear of the building in question is the contamination source, or if there may be other sources for the parasite.
Testing to gather more evidence is ongoing.
Tests taken by police have been sent to Swedish National Laboratory of Forensic Science (Statens kriminaltekniska laboratorium – SKL) for analysis and the police in Jämtland expect it to take a few weeks for the results are in.