Airline regulators in the US have formally launched an investigation of Boeing’s processes, after a door plug blew off one of its planes.
The Federal Aviation Administration (FAA) said it would examine whether Boeing failed to ensure its completed jets matched their approved design.
The FAA has already grounded most of the 737 Max 9 fleet.
Inspections following the Alaska Airlines emergency have uncovered issues such as loose bolts.
“This incident should have never happened and it cannot happen again,” the FAA said.
“Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet.”
Transportation Secretary Pete Buttigieg said earlier that the government would not be rushed into clearing the grounded Boeing 737 Max 9 planes, which has led to the cancellation of hundreds of flights.
He said the aircraft “need to be 100% safe”.
It is unclear when the planes will be allowed to fly again.
Boeing’s boss, Dave Calhoun, has described the problem revealed by the incident on the flight from Portland, Oregon to California as a “quality escape”.
It means the incident was caused by some failure in quality control in the plane, which had been in service for just eight weeks before the blowout.
Mr Calhoun told CNBC that there were still questions that need to be answered about how the incident was allowed to happen. “What broke down in our gauntlet of inspections? What broke down in the original work that allowed for that escape to happen,” he said.
Earlier this week, Mr Calhoun admitted that Boeing was at fault after a section of the fuselage from a 737 Max 9 plane operated by Alaska Airlines blew-out within minutes of take-off.
No-one was injured when the panel – or door plug – broke away from the Alaska Airlines flight from Portland, Oregon, forcing the plane to make an emergency landing.
The US Federal Aviation Administration (FAA) grounded 171 Boeing jets on Saturday that were installed with the same door plug.
The door plug is a piece of fuselage, with a window, that fills the space where an emergency exit would be in certain configurations.
Mr Buttigieg declined to say when the suspension will end.
“The only consideration on the timeline is safety,” he said. “Until it is ready, it is not ready. Nobody can or should be rushed in that process.” (BBC)