HUNTINGTON BEACH, Calif. (AP) — Authorities have detected a 2.5-mile (4 kilometer) long oil sheen off the coast of Southern California. But they said it doesn’t appear to be a crude spill and are investigating the cause.

U.S. Coast Guard Petty Officer Richard Brahm said authorities spotted the sheen after daybreak Friday about 1.5 miles (2.4 kilometers) offshore, and it later shifted about another mile further from the coast. He said there were no reports of spills or leaks from oil platforms operating offshore and the sheen does not appear to be growing, which leads authorities to suspect it was a one-time discharge or natural seepage of oil in the ocean, which is common.

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

Officials in Southern California formed a unified command to investigate the sheen, which was initially reported Thursday night, he said.

Local officials were paying close attention to the reports following a 2021 leak in an offshore pipeline that sent blobs of crude washing ashore in Huntington Beach, a surf-friendly city about 30 miles (48 kilometers) southeast of Los Angeles.