WASHINGTON (Reuters) -The U.S. National Transportation Safety Board said on Friday it is investigating why a Boeing 737 MAX operated by Southwest Airlines rolled during a flight last month.

The NTSB said the plane experienced what the crew said was a "Dutch roll" at 34,000 feet while en route from Phoenix, Arizona to Oakland, California on May 25. Such lateral asymmetric movements are named after a Dutch ice skating technique and can pose serious safety risks.

The board said pilots regained control of the plane, landed it safely and no one among the 175 passengers and six crew were injured during the incident. In a subsequent inspection, Southwest found damage to structural components, the NTSB said.

The Federal Aviation Administration (FAA), which said damage was seen in a standby power-control unit, said it is also investigating and working "closely with the NTSB and Boeing to investigate this event".

Boeing declined to comment on the Dutch Roll incident, referring questions to Southwest, which said it is participating in the investigation.

Separately, the FAA confirmed a Bloomberg News report that it is investigating a Southwest 737 MAX 8 passenger flight in April that came within 400 feet of the ocean off the coast of Hawaii after weather conditions prompted pilots to bypass a landing attempt.

Southwest said in a statement to Reuters "through our robust Safety Management System, the event was addressed appropriately as we always strive for continuous improvement."