CLINTON — Police in Clinton are trying to figure out the source of contamination in the city's water system that led to a boil order. They believe they've narrowed the problem down to a single home.

To water the lawn in Clinton, residents use a secondary water system. If it’s not turned on yet, people use a hose connected to the house's culinary water.

But investigators say instead someone routed their house water into a sprinkler system linked to the irrigation water, leading to a problem. Secondary and culinary water systems should remain separate.

"Culinary water is tested frequently and is safe to drink,” said Ken Bausfield with the Division of Drinking Water at the Department of Environmental Quality. “That is not the case with the secondary water system."

Bausfield said since irrigation water has its limits, people sometimes get other ideas.

"Perhaps the most common is they want to water their lawn, and the pressure irrigation system has not been energized, so they connect to the culinary system to water their lawns,” he said.

What people don't realize is that can contaminate the water not just for one home but for everyone in the city, he said.

"But when the pressure irrigation system is turned on, that's when the backflow event occurs,” he said.

Bausfield said that secondary water gets pushed into the house system and back out the main line in the street. “That water is then commingled with water already in the pipe and served to other neighbors,” he said.

That's likely what happened in Clinton just over a week ago. Investigators say it came from a 20-year-old house that was recently sold. They're not sure whether a previous owner may have connected the valves.

This incident is still under investigation. Clinton City Manager Dennis Cluff said when the investigation is complete, the case will be turned over to prosecutors.

The water boil order was issued June 6 after samples of city water tested positive for E. coli bacteria.

In August 2014, the city’s water tested positive for E. coli after another home had the same issue with its secondary and culinary water. This case was turned over to the county prosecutor, Cluff said.

Since this is the second time this has happened in less than a year, the department is asking the city to improve its education efforts and stiffen the penalty. The City Council is expected to discuss those potential changes soon.