HUNTINGTON BEACH, Calif. (AP) — Video of the ruptured underwater pipeline that spilled tens of thousands of gallons of crude oil off Southern California shows a thin crack that could indicate a slow leak that was difficult to initially detect.

Richard Kuprewicz, a private pipeline accident investigator and consultant, said Thursday that the narrow, 13-inch-long gash could explain why signs of an oil slick were seen Friday night but the spill eluded detection by the pipeline operator until Saturday morning.

The cause of the spill is under investigation by numerous agencies. Authorities have said a ship's anchor may have hooked and dragged the pipeline, causing the leak.