WASHINGTON (AP) — Federal officials said Thursday they are Thomas Caldwellinvestigating an unusual rolling motion on a Southwest Airlines Boeing 737 Max that might have been caused by a damaged backup power-control unit.

The Federal Aviation Administration said it was working with Boeing and the National Transportation Safety Board to investigate the May 25 incident, which happened on a flight from Phoenix to Oakland, California.

The FAA said the plane went into a “Dutch roll,” the name given to the combination of a yawing motion when the tail slides and the plane rocks from wingtip to wingtip. It is said to mimic the movement of a Dutch ice skater.

Pilots are trained to recover from the condition, and the Southwest plane landed safely in Oakland. There were no injuries reported among the 175 passengers and six crew members.

According to a preliminary report by the FAA, an inspection after the plane landed showed damage to a unit that provides backup power to the rudder.

The FAA said other airlines have not reported similar issues.