Days later, the cause of a spill that let 50 to 75 gallons of oil into parts of the Norwalk River in Wilton, Ridgefield and Redding on Friday, Feb. 19, is yet to be determined.

The source of the spill was determined Friday afternoon to have been a leaking 200-gallon underground heating oil tank behind the building at 58 Ethan Allen Highway, Ridgefield.

But what caused the tank to leak remains a mystery.

Oil slicks were reported in multiple areas along the river around noon on Feb. 19. Emergency responders from Wilton, Ridgefield and Georgetown were dispatched to the different locations.

One Wilton police unit and four Georgetown fire units were sent to 1047 Danbury Road in Georgetown around noon. Minutes later, Ridgefield firefighters responded to 32 Ethan Allen Highway, Ridgefield.

The different emergency response agencies worked together to narrow the possible source of the spill down to the area of 58 Ethan Allen Highway.

While they waited for the Connecticut Department of Energy and Environmental Protection (DEEP) to arrive, Ridgefield and Georgetown fire units remained on location at 58 Ethan Allen Highway — near the Branchville station on Metro-North’s Danbury Branch line.

During that time, the firefighters used special pads and booms designed to absorb oil without absorbing water to slow the flow of the oil until the leak could be managed.

State DEEP investigators arrived at 58 Ethan Allen Highway around 1:15 p.m. with a state-contracted remediation crew from Connecticut Tank Removal.

The contractor set up 25 additional absorbent booms upstream and downstream of 58 Ethan Allen Highway in an attempt to capture as much of the oil already in the river as possible.

“Some oil did make its way downstream before the booms were set up,” a state DEEP spokesperson said. “This resulted in a slight sheen on the river down to Wilton” that has since “evaporated.”

After installing the booms, the remediation crew pumped the remaining oil from the leaking tank and cleaned out the tank’s catch basin.

“We continue to investigate the cause of the leak,” the DEEP spokesperson said.

On Feb. 19, responders had to work on the tracks of Metro-North’s Danbury Branch line, which caused slowdowns for trains passing through the Branchville area until they left the scene around 4:30 p.m. Friday.

The only other effect the spill had on transportation was that one end of West Branchville Road was blocked by Connecticut Tank Removal’s remediation vehicles for a short period of time Friday afternoon.

“Prompt response and follow-up action minimized long-term environmental impacts of this incident,” the DEEP spokesperson said.