A potentially hazardous substance detected in the sewers of a Swansea neighborhood prompted the evacuation of 52 houses Thursday.

Emergency responders don’t yet know what the substance is or where it is coming from, but based on initial readings, it could be considered flammable and/or explosive, said Bryan Whitaker, assistant director of the St. Clair County Emergency Management Agency and public information officer for the Swansea Fire Department.

He said the fire department was first dispatched to the 1600 block of N. 16 St. at 3:55 p.m. to investigate a ‘suspicious odor’ within a residence.

That investigation led to the discovery of “a potentially hazardous substance within the sewers,” according to a Facebook post from the St. Clair County Emergency Management Agency.

Upon arrival, the fire department determined the need to request further assistance from St. Clair County Emergency Management and the St. Clair Special Emergency Services hazmat team, Whitaker said.

Those teams came out, and based on initial readings in the sewer lines in the area, they determined the need to evacuate 52 homes.

They have been working with the Illinois Environmental Protection Agency and received authorization to do a controlled flush of the sewer line.

“At this point we’re working through that controlled flush and taking readings in the sewer lines to see if the levels are dissipating or remaining the same,” Whitaker said.

Residents have self-evacuated to other people’s homes or other places, he said, and the village is making long-term plans in the event the evacuation would continue overnight. There are no injuries, he confirmed.