Bolts that need "tightening" have been found on some Boeing 737 Max 9s, the model of plane where a panel blew off mid-flight.
United Airlines said the "installation issues" relating to door plugs found on some of its planes would be "remedied" before the aircraft type would return to service.
It came as US aircraft regulators, the US National Transportation Safety Board (NTSB), said late on Monday it could not yet tell whether a recovered cabin panel that blew off an Alaska Airlines Boeing 737 MAX 9 plane in mid-air last week had been properly attached.
Nearly 200 of the planes were grounded after a panel blew off an Alaska Airlines-operated flight not long after taking off from a Portland, Oregon, airport on Friday, forcing pilots to scramble to land the plane safely.
Alaska Airlines has said it has since found "loose hardware" on some of the planes.
Despite the horrifying scenes, with passengers sharing footage of the gaping hole in the side of the plane, nobody was injured.
Passengers told local media that a young boy seated near the affected area had his shirt ripped off by the force of the decompression as his mother clung on to him.
The panel, which covers an optional exit door, was recovered on Sunday by a Portland schoolteacher who found it in his backyard.
The large majority of Boeing 737 Max 9s used in the US are operated by United Airlines and Alaska, both of whom said they had cancelled flights in the wake of the incident.
Boeing said it was staying in close contact with Max 9 operators and would help customers address any findings during inspections.
"We are committed to ensuring every Boeing aeroplane meets design specifications and the highest safety and quality standards," the planemaker said.
“We regret the impact this has had on our customers and their passengers."
Alaska Airlines cancelled 141 flights, or 20% of its scheduled departures, on Monday after grounding the planes, with disruption expected throughout the week, while United cancelled 226 flights.
The incident heaps increased pressure on Boeing, which has suffered from numerous production issues since a wider grounding of the 737 Max family in March 2019 after two deadly crashes killed 346 people.
Both crashes were caused by faulty software on the planes, which forced them into dives, despite the best efforts of the pilots.
