The investigation of a Boeing 737-9 MAX in Portland, Oregon after fuselage fell from the plane during an Alaska Airlines flight. Photo: NTSB / AFP
Bolts in need of "additional tightening" have been found during inspections of Boeing 737 Max 9s, United Airlines has said.
Inspections began after a section of the fuselage fell from an Alaska Airlines 737 Max 9 on Friday.
United Airlines said "installation issues" relating to door plugs would be "remedied" before the aircraft type would return to service.
Some 171 planes of the same type remain grounded by the US regulator.
In its statement, United said: "Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug - for example, bolts that needed additional tightening."
The door plug is a piece of fuselage with a window that can be used as an emergency exit in certain configurations.
It was this part of the Alaska Airlines plane which dramatically fell off mid-flight over the US state of Oregon, eventually landing in a teacher's back garden.
The plane made an emergency landing and none of the passengers were injured.
The vast majority of Boeing 737 Max 9s used in the US are operated by United Airlines and Alaska, while Turkish Airlines, Panama's Copa Airlines and Aeromexico have also grounded jets of the same model for inspections.
United said it had cancelled 200 flights as of Monday and expected significant cancellations on Tuesday.
"We have been able to operate some planned flights by switching to other aircraft types, avoiding about 30 cancellations each on Monday and Tuesday," United added.
- This story was first published by the BBC