United Airlines, the other US 737 MAX 9 operator, separately said it had canceled all its MAX 9 flights on Wednesday and told pilots it expected “meaningful cancellations” on Thursday.

The US Federal Aviation Administration (FAA) on Saturday grounded 171 Boeing jets installed with the same panel after the emergency landing, including Alaska’s 65 MAX-9s, that have forced the cancelling about 20% of its daily flights.

“We will only return these aircraft to service when all findings have been fully resolved and meet all FAA and Alaska’s stringent standards,” Alaska Airlines said, noting it still needs revised inspection and maintenance instructions from Boeing that must be approved by the FAA before it can begin flying the planes again.

Boeing CEO Dave Calhoun told CNBC separately on Wednesday a “quality escape” was at issue in the MAX 9 cabin blowout that left a gaping hole in the plane that had just been in service for eight weeks but added there are key questions.

“What broke down in our gauntlet of inspections? What broke down in the original work that allowed for that escape to happen?” Calhoun said.

He said a quality escape is a “a description of what people are finding in their inspections… anything that could potentially contribute to an accident.”

Calhoun said he is in talks with US transportation secretary Pete Buttigieg and FAA Administrator Mike Whitaker to ensure there is no repeat of any cabin panel blowout on any 737 MAX 9 after Friday’s incident.

Alaska Airlines and United Airlines said on Monday they had found loose parts on multiple grounded aircraft, raising new concerns among industry experts about how Boeing’s best-selling jet family is manufactured.

The US National Transportation Safety Board (NTSB) is also focused on whether the recovered cabin panel that blew off had been properly attached or if the bolts were actually present.