RENTON, WA (AP) – The FAA is investigating an unusual rolling motion that occurred during the flight of a Southwest Airlines Boeing 737 Max.

The agency said it’s working with Boeing and the National Transportation Safety Board (NTSB) on the investigation.

The incident happened May 25 on a Southwest flight from Phoenix to Oakland, California. The crew regained control of the plane and landed safely in Oakland. No injuries were reported.

The FAA said that an inspection of the Boeing Max showed damage to a unit that controls backup power to the plane’s rudder.

No other airlines have reported a similar problem.