The U.S. Coast Guard on Friday said it was investigating an oil sheen that appeared to be nearly 2.5 miles long and half a mile wide, off the coast of Huntington Beach, Calif. It was not clear what caused it.

The Coast Guard received a National Response Center report around 7 p.m. on Thursday of an “unknown substance in the water” about 1.5 miles off the coast, Richard Brahm, a spokesman for the Coast Guard, said.

The Coast Guard’s pollution responders, the California Department of Fish and Wildlife, Huntington Beach’s lifeguards and the Orange County Sheriff’s Office investigated through the night and into the morning. The investigation was still underway late Friday morning.

“Right now we’re trying to figure out if there are any potential sources for the spill,” Mr. Brahm said, adding that there were offshore oil platforms in the area, but it was too early to know what caused it. “It could be natural seepage, it could have been a discharge. We’re not sure.”

Mr. Brahm said the goal was to “prevent this from impacting the environment as much as possible, and get it cleaned up.” He said that normally when spills were offshore and not stemming from a continuous stream or source, they broke up on their own.

The U.S. Coast Guard of Southern California said in a social media post that officials planned to use aerial surveys “to assess the size and potential impacts.”

Katrina Foley, a supervisor for Orange County, said on social media that it was possible the oil spilled from an offshore platform, but Mr. Brahm called that possibility “speculation.” In the past, when there have been spills of that nature, the companies operating the platforms have sent a report to the Coast Guard and other officials.

“If it is one of the platforms, obviously we’ll hold them responsible and work with them,” Mr. Brahm said. “They’ll be the responsible party, and they’ll get it cleaned up, but right now it’s all under investigation and we’re trying to figure it out.”

In 2021, a pipeline failure off the coast of Orange County caused tens of thousands of gallons of oil to spill into the Pacific Ocean, killing fish and birds and infiltrating the nearby Talbert Marsh. At that time, a 13-mile slick extended from Huntington Beach to Newport Beach. The spill involved a failure in a 17.5-mile pipeline connected to an offshore oil platform operated by Beta Offshore.

Governor Gavin Newsom of California said on social media on Friday that state officials were “actively monitoring” the situation.

“We have deployed personnel to evaluate the incident and will continue to coordinate with local, state and federal partners,” he wrote.