
United Airlines has said that during inspections of the "Boeing 737 Max 9" plane, bolts were found that needed "additional reinforcement", reports the BBC.
The inspections began after the door of the Alaska Airlines 737 Max 9 was detached on Friday while it was in the air.
United Airlines said "installation issues" related to the doors will be "resolved" before the aircraft type returns to service.
About 171 planes of the same type have been grounded by the regulator in the US.
"Since the initial inspections began on Saturday, we have found instances that appear to be related to an installation issue in the door jamb — for example, bolts that needed additional reinforcement," United Airlines said in a statement.
The door of the "Alaska Airlines" plane fell dramatically mid-flight over Oregon.
It had fallen in a teacher's garden. The plane made an emergency landing and there were no injured passengers.
The vast majority of Boeing 737 Max 9 aircraft in the US are operated by United Airlines and Alaska Airlines. While Turkish Airlines, Copa Airlines and Aeromexico have also decided to suspend flights of this model until inspections are carried out.
About 200 United Airlines flights were canceled on Monday and more cancellations are expected on Tuesday.
"We were able to make some flights by switching to other types of aircraft," United Airlines said.