State and industry officials and the U.S. Coast Guard were responding on Tuesday to an unexplained oil spill roughly 60 miles southwest of Anchorage in Southcentral Alaska’s Cook Inlet.

The spill, which produced a long oil sheen on the water near the Trading Bay area on the western side of the inlet, was first spotted by a pilot on Monday. His report prompted a multiagency response.

The multiagency unified command is composed of the Alaska Department of Environmental Conservation, the U.S. Coast Guard and Hilcorp, the dominant oil and gas operator in Cook Inlet.

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The sheen was seen near the idled Spark platform, one of 17 oil and gas platforms in the inlet. Six of those, including Spark, are now inactive. The Spark platform was acquired by Hilcorp as part of its 2012 purchase of Marathon Oil’s Cook Inlet assets. It has been inactive since 1992, said a statement issued by the unified command responding to the spill. The wells associated with the Spark platform were plugged and abandoned by a previous owner in 2009, the statement said.

As of Tuesday afternoon, however, the ongoing investigation has not pinpointed a source of the spill, its size or even the substance, DEC officials said.

An oil-response vessel sent by Hilcorp to the site arrived shortly after 2 a.m. on Tuesday, but its crew was not able to spot the sheen with infrared imaging technology, said the incident command statement. The spill-response vessel, the Endeavor, has been sailing in a grid pattern to search for the spilled material and its possible source, the statement said.

There were no reports of impacts to fish or wildlife as of Tuesday afternoon, said a DEC situation report issued on Tuesday afternoon.

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Fish could be potentially at risk, the situation report said. All five species of Pacific salmon swim in the affected waters, and coho and sockeye salmon were migrating through the area at the time the spill was reported, the situation report said.

Representatives of area environmental groups said Tuesday that the event was a warning about risks to the inlet.

Satchel Pondolfino, the clean water lead for Cook Inletkeeper, said she and others are concerned about the apparent association with the idled but still-standing Spark platform.

“It’s just another reason why it’s so important that Alaska use its authority to hold Hilcorp and every other operator accountable for dismantling the infrastructure,” she said.

Activists have called for the removal of such unused infrastructure from the inlet, but the state so far has not ordered any company to do that.

Cooper Freeman, Alaska director of the Center for Biological Diversity, said numerous natural resources continue to be at risk from the incident and others like it.

“It’s gut-wrenching to see the inlet’s beleaguered salmon and belugas take a hit they can’t afford from a miles-long oil spill,” he said in an emailed statement. “We need Cook Inlet’s fish and wildlife to recover, and we can’t keep treating their home like an industrial waste zone. This is yet another wake-up call that our over-reliance on fossil fuels imperils not just our region’s long-term energy stability, but the vitality of our environment for all future Alaskans.”