The finding from technicians comes after United Airlines reported it found issues relating to the installation in the door plug.
Alaska Airlines has said more “loose hardware” has been found on some of its Boeing 737 MAX 9 fleet of aircraft.
It follows the Federal Aviation Administration (FAA) grounding all 171 737 MAX 9 planes operated by US airlines after a window and chunk of fuselage blew out of one Alaska Airlines aircraft on 5 January shortly after it took off from an airport in Portland, Oregon.
Six crew members were seriously injured after the door plug – used to replace an exit that would be installed on planes configured to carry more passengers – tore off around six minutes into the flight to Ontario, California, causing depressurisation and forcing pilots to turn back.
In its latest statement the airline said: “Initial reports from our technicians indicate some loose hardware was visible on some aircraft”.
It added that all aircraft were being “thoroughly inspected in accordance with detailed instructions provided by the FAA in consultation with Boeing”.
On Monday, United Airlines reported it has found loose bolts on plug doors on multiple 737 Max 9 aircraft during inspections.
So far United found nearly 10 planes with loose bolts during its preliminary checks, according to a source, up from an initial five first reported by industry publication The Air Current, and the figure may increase.
Earlier, investigators said crew on the plane that lost its door plug had reported that the auto-pressurisation fail light lit up on the same aircraft on 7 December last year and 3 and 4 January this year.
After those warnings, the airline chose to ban the aircraft from making long flights over water to Hawaii, in case it needed to turn back to an airport, the National Transportation Safety Board (NTSB) said.
The NTSB has said it is unclear if there is any connection between those incidents and the accident, but it will be able to determine if the door plug that flew out was properly bolted to the fuselage, or indeed if bolts existed.
It said it was also examining the possibility of a systemic issue with the aircraft type.
NTSB chair Jennifer Homendy said: “We are not shy about going broader than just this aircraft, but right now we are focusing on just this one, we have to figure out if this is a wider fleet issue.”
Alaska Airlines said travel disruptions on its services are expected until at least mid-week after grounding 20% of its scheduled flights.
United also cancelled 226 flights on Monday.
While the airlines affected and their passengers are suffering disruption, the Alaska Airlines incident has shone a further light on standards at Boeing and within its supply chain.
The company, whose Max 8 planes were grounded globally in 2019 after two fatal crashes, saw its shares lose 8% on Monday.
Those of Sprit Aerosystems, which made the blowout part and is reported by US media to have made the initial installation, fell by 11%.
‘It is in my backyard!’
Following the incident on 5 January, US authorities asked people in the Cedar Hills suburb to look out for the missing door plug – a vital piece of evidence.
Two days later, Bob Sauer, a science teacher, reported something “gleaming white” underneath the trees in his garden, which turned out to be the mid-cabin door plug.
“It was very obviously part of a plane. It had the same curvature as a fuselage, it had a plane type window in it, and it was white,” he said.
“My heart did start beating a little fast at that point because I thought: Oh my goodness, people have been looking for
this all weekend and it looks like it is in my backyard!” Mr Sauer said.
The panel has been sent to a NTSB lab in Washington for further examination, having miraculously remained undamaged by the fall.
Mr Sauer said the trees had acted like an airbag.