Low levels of potentially harmful chemicals used in firefighting foam and manufacturing were found in drinking water in suburban Denver, health officials said Thursday.

Compounds called PFCs were detected in wells used by the South Adams County Water and Sanitation District, the Colorado Department of Public Health and Environment said. The district serves about 50,000 people in Commerce City.

Untreated water in 12 wells had PFC concentrations between 24 and 2,280 parts per trillion, said Kipp Scott, the district’s water system manager. After treatment at the district’s plant, PFC levels in water delivered to customers ranged from 45 to 64 parts per trillion, below the federal government’s advisory limit of 70 parts per trillion, he said.

The water district said its water is safe.

Three wells with the highest concentration were shut down, and the district is buying water from Denver’s utility to make up the difference, Scott said.

With those wells shut off and Denver water added to the system, “we expect treated water levels to be much lower,” he said.

Federal, state and local agencies are looking for the source of the PFCs and checking other wells, officials said.

Some kinds of PFCs have been linked to prostate, kidney and testicular cancer, along with other illnesses. PFCs were widely used in non-stick coatings on cookware as well as in firefighting foam and other applications. Some manufacturers have agreed to phase them out.

PFCs are not a regulated chemical, but the U.S. Environmental Protection Agency set the advisory limit of 70 parts per trillion in 2016.

Commerce City is the second Colorado location known to have PFCs in its well water.

In 2015, the compounds were found in three utility systems serving about 69,000 people south of Colorado Springs in the city of Fountain and in Security-Widefield, an unincorporated community.

PFC levels there exceeded EPA advisory limits. The utilities took steps to reduce the amount of water they took from the contaminated wells or shut them down.

The PFCs were traced to nearby Peterson Air Force Base, which had used firefighting foam containing the compound.

The Air Force agreed to buy clean drinking water and to operate and maintain filter systems for the utilities.