US carriers United and Alaska both said they found loose bolts and hardware on grounded Boeing 737 Max 9 planes in their fleets.

"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," Chicago-based United Airlines said.

The plug closes a hole that could be used for an extra emergency exit door on planes configured with relatively high numbers of seats.

Alaska Airlines also cited its technicians as indicating in initial reports that "loose hardware" was visible on some aircraft in the relevant area when it conducted checks of its fleet.

The airline said it awaited Boeing's and the US Federal Aviation Administration (FAA) final documentation before it could begin formal inspections.

The discovery comes after a door plug blew out on an Alaska Airlines 737 Max 9 on Friday, alarming regulators both in the US and abroad.

Both United Airlines and Alaska Airlines are primary users of the 737 Max 9 aircraft.

The fleet's grounding and its repercussions

The US FAA approved on Monday guidelines for inspecting the door plugs on other Max 9 jets and repairing them when needed.

The decision could facilitate the return to the skies of some 171 planes which the FAA grounded over concerns following the Friday incident.

Of the grounded planes, Alaska Airlines has some 65, while United Airlines owns 79, making them the only two US airlines operating the Boeing model in question.

The grounding affected Boeing's shares, which fell 8% when trading returned for the first time since the incident on Monday. The shares of Spirit AeroSystems, which builds the fuselage for Boeing's 737 Max, also slumped 11%.

What was the mid-air scare?

An Alaska Airlines flight had to make an emergency landing when a window and piece of fuselage blew out in midair on Friday.

The hole in the aircraft was ripped open some 20 minutes after takeoff, causing the cabin to depressurize.

Oxygen masks were released and the plane safely landed soon after, with over 170 passengers and six crew members unharmed.

Grounded until 'enhanced inspections'

The FAA on Saturday ordered US airline operators to temporarily ground some Boeing 737 Max 9 aircraft for inspections.

It said planes would remain grounded "until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners."

The Boeing 737-9 MAX just received its certification last October, FFA online records show. It has been on 145 flights since going into commercial service on Nov 11.

The twin-engine, single-aisle MAX is the newest version of most-flown commercial series of aircraft in the world, Boeing 737s. In service since May 2017, the Max is frequently used on US domestic flights.

Two MAX 8 aircraft crashed in 2018 and 2019, killing 346 people and prompting a worldwide grounding of all Max 8 and Max 9 planes that lasted nearly two years.