WASHINGTON — The Coast Guard has asked the Justice Department to investigate possible pollution violations by both the drilling rigs Shell used in its botched efforts to explore for oil last year in the Arctic Ocean waters off the northern coast of Alaska.

Coast Guard Rear Adm. Thomas Ostebo said Wednesday that he’d turned over to the Justice Department for review and possible prosecution an investigation into the troubled Shell drilling rig Kulluk.

The Coast Guard earlier had sent the Justice Department a list of 16 safety and environmental violations by the other rig used in Shell’s Arctic efforts, the Noble Discoverer.

“As the Coast Guard and Department of Justice are still actively engaged in these investigations, it would not be appropriate for me to provide additional information at this time,” Ostebo said at a Senate hearing in Anchorage, chaired by Alaska Democratic Sen. Mark Begich.

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Shell Alaska Vice President Pete Slaiby said at the hearing that he also wouldn’t discuss any matters under investigation, but he defended Shell’s efforts.

The Kulluk was grounded for several days off Kodiak Island after a New Year’s Eve storm. Ostebo, who’s the Coast Guard’s commander for Alaska, said the grounding was an “event that highlights the rigors of operating in Alaskan waters.”

Ostebo said the Coast Guard was investigating the Kulluk grounding with help from the National Transportation Safety Board and the Bureau of Safety and Environmental Enforcement.

The Noble Discoverer was found to have 16 violations after a Coast Guard inspection in November. The violations included pollution-control problems and a finding that the vessel couldn’t go fast enough to maneuver safely in rough Arctic conditions.