A door panel that flew off a Boeing 737 MAX 9 jet mid-flight appeared to be missing four key bolts, according to an initial report by a US safety board investigating the incident.
Photo evidence released Tuesday shows bolts were missing from the door plug, which had been removed to fix rivets which were damaged in the production process, according to the independent US National Transportation Safety Board (NTSB) report.
Until now, the NTSB had not said what caused the panel to rip off an Alaska Airlines-operated jet as the plane climbed to 16,000 feet on January 5.
The incident has become a full-blown safety and reputational crisis for Boeing and will slow plane production.
Boeing CEO Dave Calhoun said the company was accountable, whatever the findings may reveal.
"An event like this must not happen on an airplane that leaves our factory," Mr Calhoun said.
"We simply must do better for our customers and their passengers."
The US Federal Aviation Administration grounded 171 of the Boeing 737 MAX 9 planes after the incident, most operated by US carriers United and Alaska Airlines, for inspections.
Those planes were cleared to return to service in late January.
The NTSB has been focused on how the panel — fitted into this MAX 9 model in place of an optional exit — detached from the plane.
The plug is held down by four bolts and then secured by "stop fittings" at 12 different locations along the side of the plug and the door frame.
All 12 stop fittings became disengaged during the event, the NTSB said in January.
Both United Airlines and Alaska Air said in the days after the blowout they had found loose parts on multiple grounded MAX 9 aircraft.
The plug was manufactured by Spirit AeroSystemsSPR.N, the one-time subsidiary of Boeing which separated from its parent in 2005.
'Current system not working', FAA administrator says
The Federal Aviation Administration (FAA) has taken a hard line on Boeing.
In late January, it barred Boeing from expanding production of its 737 MAX planes due to the quality issues.
That means it can continue producing MAX jets at its current rate, but it cannot increase that rate.
"I certainly agree that the current system is not working, because it's not delivering safe aircraft," FAA administrator Mike Whitaker told politicians on Tuesday.
"So we have to make changes to that."
US Representative Rick Larsen, overseeing the FAA, said the "failure to re-install bolts on a safety-critical component of this 737 MAX 9 aircraft is a serious error that signals larger quality control lapses that must be corrected."
The FAA is conducting an audit of 737 MAX manufacturing, which is looking at all elements of production at Boeing and fuselage production at its supplier Spirit.
Boeing shares were up 1.6 per cent in trading Tuesday.
The stock has lost more than 20 per cent of its value since the beginning of the year.
Reuters