United Airlines says it has discovered loose bolts on the door plugs in several of its grounded 737 Max 9 planes days after a door plug dramatically blew out of an Alaska Airlines plane mid-flight. 

“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,” United said in a statement Monday US time.

“These findings will be remedied by our Tech Ops team to safely return the aircraft to service,” said the US airline, which has 79 Boeing 737 MAX 9 planes in its fleet.

The door plug, a large piece of the plane’s fuselage, tore off the left side of an Alaska Airlines jet following take-off from Portland en route to Ontario, California on Friday, forcing pilots to turn back and land safely with all 171 passengers and six crew on board, with only a few minor injuries reported.

The panel, put in place on some planes in lieu of an additional emergency exit, was eventually found in a suburban backyard Monday by a Portland schoolteacher identified only as ‘Bob’.

National Transportation Safety Board (NTSB) chair Jennifer Homendy said she was “very relieved” it had been found.

She had earlier told reporters the aircraft part was vital to determine why the accident occurred.

“Our structures team will want to look at everything on the door — all of the components on the door to see to look at witness marks, to look at any paint transfer, what shape the door was in when found. That can tell them a lot about what occurred,” she said earlier.

The force from the loss of the plug door was strong enough to blow open the cockpit door during flight, said Homendy.

United, which did not confirm how many planes had loose bolts on its door plugs, said it was working on returning the aircraft into service as soon as possible but expected “significant cancellations”.

It has already canceled 200 MAX 9 flights since an Alaska Airlines Boeing 737 Max 9 plane suffered a mid-air incident Friday evening that required an emergency landing.

“As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings,” Boeing said in a statement Monday.

“We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers.”

The Federal Aviation Administration (FAA) did not rule out further action as a probe began into the apparent structural failure.

Investigators will look at maintenance records, the pressurisation system and the door components.

Alaska Air said it had halted flights by 18 of its MAX 9 planes that it had resumed using on Saturday after recent in-depth inspections.

The airline said it was in discussions with the FAA “to determine what, if any, further work is required before these aircraft are returned to service”.