The U.S. Coast Guard on Friday was trying to identify the source of an oil spill off the coast of Huntington Beach, California, the agency said.

The sheen, estimated at 4 kilometers in length and 800 meters in width, was discovered after a Coast Guard helicopter flew over the site at sunrise.

The spill is located roughly 4.5 kilometers off Huntington Beach, near two oil drilling platforms but it was unclear whether either were the source of the spill, according to the Coast Guard's statement.