A Boeing 737 Max suffered damage to parts of the plane’s structure after it went into a “Dutch roll” during a Southwest Airlines flight last month, U.S. investigators said Friday.

The flight took place May 25, but Southwest did not notify the National Transportation Safety Board about the roll or damage to the jetliner until June 7, the NTSB said.

The NTSB comment suggests the incident was more serious than previously known.

“Following the event, SWA performed maintenance on the airplane and discovered damage to structural components,” the safety board said.

A Dutch roll is an unstable and potentially dangerous combination of yaw, or the tail sliding sideways, and the plane rocking from side to side. The motion repeats, usually several times.

Pilots train to recover from a Dutch roll, and most modern planes include a device called a yaw damper that can correct the situation by adjusting the plane’s rudder. A preliminary report by the Federal Aviation Administration said that after the Southwest plane landed, damage was discovered to a unit that controls backup power to the rudder.

The NTSB said it downloaded data from the plane, a Boeing 737 Max 8, which will help investigators determine the length and severity of the incident.

Investigators won’t know precisely what the pilots were saying, however: The cockpit voice recorder was overwritten after two hours.

The plane was heading from Phoenix to Oakland, California. Pilots regained control and landed at Oakland. There were no reported injuries on the flight, which carried 175 passengers and a crew of six.