A Mediterranean Shipping Co (MSC) containership has been designated a "party in interest" in the investigation into an oil spill off the coast of California.

The 13,200-teu MSC Danit (built 2009) was boarded by US Coast Guard (USCG) and National Transportation Safety Board (NTSB) marine casualty investigators on Saturday in the Port of Long Beach.

Prior to the visit, the local USCG’s chief of investigations determined that the containership was involved in a 25 January 2021 anchor-dragging incident during a heavy weather event that impacted the ports of Los Angeles and Long Beach.

"The anchor-dragging incident occurred in close proximity to a subsea pipeline, which was subsequently discovered to be the source of the Orange County oil spill on 2 October 2021," the USCG said.

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, USCG spokeswoman Sondra-Kay Kneen told Associated Press.

"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.

Amplify Energy, which owns the pipeline, had previously said that it had been "pulled like a bowstring" from where it should have been.

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 pre-existing problem, Kneen added.

The party in interest designation provides the owner and operator of the MSC Danit the opportunity to be represented by counsel, to examine and cross-examine witnesses, and to call witnesses who are relevant to the investigation.

Despite the boarding, the USCG said its marine casualty investigation into the Orange County oil pipeline major marine casualty remains "ongoing" and that "multiple pipeline scenarios and additional vessels of interest continue to be investigated".

At least two other containerships, including one owned by Hapag-Lloyd, have previously been boarded by investigators as part of the investigation.

The amount of crude oil spilled from the pipeline leak is believed to be close to 25,000 gallons, or only about one-fifth of what officials initially feared, a USCG official has said.

MSC has been contacted for comment.