Investigators believe a 1,200-foot cargo ship dragging anchor in rough seas caught an underwater oil pipeline and pulled it across the seafloor, months before a leak from the line fouled the Southern California coastline with crude.

A team of federal investigators trying to chase down the cause of the spill boarded the Panama-registered MSC DANIT just hours after the massive ship arrived this weekend off the Port of Long Beach, the same area where the leak was discovered in early October.

During a prior visit by the ship during a heavy storm in January, investigators believe its anchor dragged for an unknown distance before striking the 16-inch steel pipe, Coast Guard Lt. j.g. SondraKay Kneen said Sunday.

The impact would have knocked an inch-thick concrete casing off the pipe and pulled it more than 100 feet , bending but not breaking the line, Kneen said.

Still undetermined is whether the impact caused the October leak, or if the line was hit by something else at a later date or failed due to a preexisting problem, Kneen said.

“We're still looking at multiple vessels and scenarios,” she said.

The Coast Guard on Saturday designated the owner and operator as parties of interest in its investigation into the spill, estimated to have released about 25,000 gallons of crude into the water, killing birds, fish and mammals.