Authorities in southern California are investigating a 2.5-mile (4km) long oil sheen that emerged off the coast of Huntington Beach on Thursday evening.

The sheen does not appear to be a crude oil spill – it could be the result of natural seepage – and federal and state teams have deployed to the area to determine the source.

“We are going all out to find out what this is because we don’t know what this is,” said Richard Brahm, a US Coast Guard petty officer. “As far as we can tell, it’s not a spill.”

The coast guard first received a report of an “unknown substance” in the water on Thursday evening after dark and sent out a team via boat and helicopter at first light on Friday morning, according to a coast guard statement.

Authorities spotted the sheen after daybreak on Friday about 1.5 miles (2.4km) from the shore of Huntington Beach, though it later shifted another mile from the coast. The oil patch was near two offshore oil platforms, but Brahm said there were no reports of spills or leaks. Authorities suspect it was a one-time discharge or natural seepage of oil in the ocean, which is common, as the sheen does not appear to have grown.

The oil sheen fueled concern in Huntington Beach, a surf city about 30 miles (48km) from Los Angeles, where a 2021 leak in an offshore pipeline spilled 25,000 gallons (95,000 liters), covering the shore in sticky black globules and killing wildlife. The company that owned that pipeline, Amplify Energy Corp, said in a statement on Friday that it has “no indication” the sheen is connected to its operations.

The governor’s office of emergency services confirmed it had deployed teams to the area to assist the coast guard and other local personnel in their response. The California department of fish and wildlife is investigating and seeking to determine whether or not the sheen is the result of a spill, state senator Dave Min said in a statement.

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Authorities have not spotted any oiled wildlife, the coast guard said. Although the city of Huntington Beach said it had received reports of tar balls on part of the shore, there are no beach closures in effect.