(CNN) For 10 years, some residents in Denmark, South Carolina, have been suspicious of the rust-colored water coming from their taps. They’ve been collecting samples in jars and using bottled or spring water, even though the local and state government assured them it was safe.

But through a Freedom of Information Act request and a one-year investigation, CNN has found new information that may cast doubts on those assurances.

The state government was adding a substance to one of the city’s four wells, trying to regulate naturally occurring iron bacteria that can leave red stains or rust-like deposits in the water. The substance, known as HaloSan, was not approved by the US Environmental Protection Agency to disinfect drinking water.

The city’s mayor says that all of the city’s wells flow into one system to be distributed throughout the city.

The EPA and the state of South Carolina have confirmed to CNN that there is now an open investigation into how this happened, although neither would comment on the target of the probe or the scope.

It’s unclear what the effects of HaloSan might have been on the almost 3,000 people who live in this rural, tight-knit community, but a group of about 40 residents believe the water is to blame for illnesses and maladies they say they’re suffering from.

The chemical is typically used as a disinfectant for pools and spas, but several experts contacted by CNN said they could not find another instance where it was added to a drinking water system.

One thing is clear — the state of South Carolina approved its use, and it should not have. It was used for 10 years.

CNN was told by the state that it has been adding HaloSan to the water in Denmark since 2008.

A spokesman for South Carolina’s Department of Health and Environmental Control told CNN in an email that it believed…read more