WASHINGTON, June 14 (Reuters) - The U.S. National

Transportation Safety Board said on Friday it is investigating

why a Boeing 737 MAX operated by Southwest Airlines ( LUV )

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 ( LUV ) 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 ( BA ) to

investigate this event".

Boeing ( BA ) declined to comment on the Dutch Roll incident,

referring questions to Southwest ( LUV ), which said it is participating

in the investigation.

Separately, the FAA confirmed a Bloomberg News report that

it is investigating a Southwest ( LUV ) 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 ( LUV ) said in a statement to Reuters "through our

robust Safety Management System, the event was addressed

appropriately as we always strive for continuous improvement."