Whoever was dumping oil into Newtown Creek this summer may have assumed that nobody would notice, given that the murky waterway between Brooklyn and Queens had been forsaken for so long.

But the stream of calls to the oil spill hotline operated by the New York State Department of Environmental Conservation proved otherwise. This week, one of those calls spurred an investigation that identified a potential culprit in the dumping, which required an extensive cleanup involving the Coast Guard, according to department officials.

That effort continued on Friday, with blue and yellow booms stretched across the creek to contain and soak up the oil. The conservation department, which also brought in a private contractor to help the Coast Guard, is “actively investigating” the source of the oil, said Peter Constantakes, a department spokesman.

The presence of oil in the creek has been revealing itself for several weeks in the form of multicolored slicks on the surface. Alarmed by these “rainbow sheens,” neighbors and environmental activists have been reporting them to the department. But nobody had a strong suggestion about the source of the spills — until this week.